Detectors, multiplexing, readout – survey of current technologiesand areas of focus for CMB-S4

September 16, 2016

DRAFT

CMB-S4 Collaboration

Executive Summary• This white paper provides an initial survey of the state of low-noise sensors and signal read-

out suitable for CMB polarimetry, focusing on promising scalable technologies for CMB-S4.This paper does not provide a comparative performance or cost review and does not attemptto rank technologies. Instead, it identifies viable R&D paths to explore and establish thefeasibility of each of these technologies for comparative review at a future time.

• We have identified Transition Edge Sensors (TES) and Microwave Kinetic Inductance De-tectors (MKID) as the leading signal transduction candidate technologies. TESes posses along record of well-characterized performance and CMB science results. They are the natu-ral choice for CMB-S4 and will benefit from production scaling R&D. MKIDs show nearlycomparable noise performance in laboratory testing and employ a simpler readout schemethan traditional TES readout schemes but are at an earlier stage of technological maturity.Adequate low frequency noise performance has been demonstrated only in a lumped elementMKID design. An on-sky CMB mapping demonstration is essential to validate MKIDs inthe field before considering as an option for CMB-S4.

• Experience from Stage 2 and Stage 3 CMB experiments indicates that cold multiplexing ofdetectors should be implemented to mitigate integration challenges and reduce heatload atsub-kelvin cryogenic stages.

– TES multiplexing will use one or more of three candidate technologies – Time-domainmultiplexing using SQUIDs as switches, frequency-domain multiplexing using in-seriesLC resonators, or frequency-domain multiplexing using microwave-interrogated res-onators. With some effort, the first two techniques are scaled to multiplexing factors of200. Further scalability requires modest to extensive R&D, but is recommended. Thethird technique could produce MKID-like high multiplexing factors for TESes ( 1000)with process R&D into resonator packing.

– MKID cold multiplexing is naturally frequency-domain, and uses microwave-interrogatedresonators that are built into the detection architecture. High multiplexing factors arereadily achieved even today, requiring minimal R&D on this front.

• All TES architectures rely on cold-stage signal amplification from SQUIDs. All microwave-interrogated techniques use a cold-stage low-noise amplifier such as a HEMT. Thus, fabri-cation of SQUIDs and cold amplifiers at the CMB-S4 scale should be investigated.

• In all cases, the warm readout electronics appear scalable with R&D. Frequency-domainmultiplexing schemes for both TES and MKID use similar room-temperature biasing andreadout electronics, enabling common development. In particular, schemes to ensure linear-ity of these systems at high multiplexing count should be validated.

1

Contents1 Introduction 3

2 Transition Edge Sensors 42.1 Technical Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42.2 Demonstrated Performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42.3 Prospects and R&D path for CMB-S4 for TESes . . . . . . . . . . . . . . . . . . 5

3 Microwave Kinetic Inductance Detectors (MKIDs) 73.1 Description of the Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73.2 Demonstrated Performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73.3 Prospects and R&D Path for CMB-S4 for MKIDs . . . . . . . . . . . . . . . . . . 8

4 Time Domain Multiplexing 114.1 Technical description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114.2 Demonstrated Performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124.3 Prospects and R&D path for CMB-S4 for TDM . . . . . . . . . . . . . . . . . . . 13

5 Frequency Domain Multiplexing 155.1 Technical description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155.2 Demonstrated Performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 175.3 Prospects and R&D path for CMB-S4 for FDM . . . . . . . . . . . . . . . . . . . 17

6 Microwave SQUIDs 206.1 Technical description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 206.2 Demonstrated Performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 206.3 Prospects and R&D path for CMB-S4 for microwave SQUIDs . . . . . . . . . . . 21

7 Room-temperature electronics for readout in the frequency domain 237.1 Description of the Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . 237.2 Demonstrated Performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 247.3 Prospects and R&D path for CMB-S4 for microwave readout . . . . . . . . . . . . 24

2

1 IntroductionCHARGE: Summarize the current state of the technology and identify R&D efforts neces-sary to advance it for possible use in CMB-S4. CMB-S4 will likely require a scale-up innumber of elements, frequency coverage, and bandwidth relative to current instruments.Because it is searching for lower magnitude signals, it will also require stronger control ofsystematic uncertainties.

In this white paper, we briefly review the state of low-noise sensors and signal readout suitablefor CMB polarimetry, focusing on scalable technologies that hold promise for CMB-S4. For alltechnologies described here, we provide (1) an overview and references for further study, (2) asummary of current performance as demonstrated on-sky for technologies with established her-itage, and laboratory performance for technologies with promising initial results (3) challengesand requisite R&D path to scale and/or refine the technology for CMB-S4 requirements.

3

2 Transition Edge Sensors

2.1 Technical DescriptionA Transition Edge Sensor (TES) is a highly sensitive thermometer consisting of a thin super-conducting film weakly heat-sunk to a bath temperature much lower than the superconductor Tc(see Fig. 1, left). By supplying a voltage bias to the TES, the sensor can operate in the middleof its superconducting-to-normal transition where small changes to the TES temperature, arisingfrom changes in the absorbed power, lead to large changes in the TES electrical resistance. Thecombination of voltage bias and sharp transition lead the TES to experience strong electrothermalfeedback [1]: the TES Joule power dissipation, V 2/R, opposes changes in the incident power,maintaining the TES at a nearly constant temperature. This negative feedback linearizes the de-tector’s response, expands its bandwidth, and ensures a simple relationship (“self-calibration”)between observed TES current and incident power.

Operationally, TES detectors are voltage biased using either an AC or DC signal and are readoutusing Superconducting QUantum Interference Devices (SQUIDs). SQUIDs have a large noisemargin over the detector noise enabling multiplexed detector readout schemes (see TDM, FDMand uMUX). Multiplexed readout is important for operating large arrays of detectors at sub-Kelvintemperatures. An important consideration in TES detector design is operational stability of theelectro-thermal circuit. The detector’s operational time constant needs to be fast relative to the skysignal, but slow relative to the readout per-channel bandwidth. Fielded TES detectors satisfy theseconstraints by engineering the detector transition shape, internal heat capacities and conductivitiesto realize operational time constants of ∼1 ms.

The theoretical foundations of TES dynamics are well developed [2] providing good descrip-tions of the noise and response for real devices. In CMB applications, the irreducible noise fora TES detector arises from statistical fluctuations in the absorbed photons [3]. For ground-basedexperiments, this noise is typicallyO(10) aW/rtHz, though values vary depending on platform/site,observation frequency/bandwidth, and the instrumental throughput/efficiency. The second sourceof fundamental noise for TES bolometers comes from fluctuations in the thermal carriers of theTES weak thermal link [4]. With appropriate thermal isolation structures and Tc ranging from 100-500 mK, TES detectors can achieve thermal conductivities of ∼80-200 pW/K, where the thermalfluctuation noise becomes comparable or subdominant to the photon noise. Together with suffi-ciently low noise readout electronics, TES bolometers have achieved nearly “background limited”sensitivities.

2.2 Demonstrated PerformanceTES detectors have been applied across a diverse set of CMB experimental platforms. Currentdetector architectures utilize low-loss superconducting microstrip coupled to planar structures torealize optical bandpass definition, polarization analysis, beam synthesis and radiation coupling(see RF coupling paper). Examples of implemented TES architectures include the antenna arrayused by the SPIDER, BICEP2, BICEP3 and Keck Array experiments, lenslet coupled antennas [5]used by the Polarbear experiment, absorber coupled devices used by the EBEX [6] and SPTpol

4

Heat Sink (~240 mK)

Weak thermal link, G-­‐1

Heat Capacity

Psignal TES

T+δT

δT

δR

Figure 1: Left: Illustration of a thermal circuit for a typical Transition Edge Sensor (TES) detectorhighlighting the principles of signal detection. A weakly thermally sunk heat capacity absorbspower, Psignal, which is to be measured. Variations in the absorbed power change the heat capac-ity’s temperature, which is measured by a TES operating under strong electro-thermal feedback.Right: Plot of resistance versus temperature for a typical TES illustrating the principles of negativeelectro-thermal feedback [1]. The TES is voltage biased into the middle of its superconducting-to-normal transition. Small changes in the TES temperature produce large changes in the TESresistance. Since the TES is voltage biased, an increase (or decrease) in the temperature producesan increase (or decrease) in the resistance leading to a decrease (or increase) in the Joule heatingpower supplied by the bias. This canceling effect corresponds to a strong negative electro-thermalfeedback making the current through the TES nearly proportional to Psignal.

(90 GHz) [7] experiments, and feedhorn coupled devices with planar orthomode transducers usedby the ABS, CLASS [8], ACTpol [9] and SPTpol (150 GHz) [10] experiments. For these detectorarchitectures, the RF performance can be modeled and simulated with results in good agreementwith measured performance (see RF paper). TES detectors have been deployed across experimentsspanning 40 GHz-300 GHz, the entire optical frequency range envisioned for CMB-S4, with de-tectors achieving NEPs of 30-50 aW/

√Hz (nearly background limited at CMB frequencies). De-

tectors deployed at low optical frequencies (∼40 GHz) and balloon-borne payloads should realizeeven lower NEPs of ∼10 aW/

√Hz. In multiple deployed experiments, the TES noise is consistent

with what is predicted from theoretical modeling with realized experimental sensitivities (arrayNET) in the range of ∼10-20 µK

√s [11, 9, 12, 13].

2.3 Prospects and R&D path for CMB-S4 for TESesGiven the maturity, diversity and demonstrated performance of TES-based CMB detectors, theTES bolometer technology is at a high technical readiness level. R&D to scale up TES arrayproduction is the most critical element in advancing TES technology for CMB-S4. TES detectorsare fabricated via micro-machining of thin films deposited on silicon wafer substrates.

5

• Increased Production Throughput Current TES detector array fabrication typically in-volves processing ∼10 layers of materials on substrates that are 100-150 mm in diameter. A150-mm wafer supports ∼1000 detectors at 150 GHz, a density which varies strongly withobserving frequency and which can be multiplied with multichroic designs. Arrays are typi-cally fabricated by a team of 2-3 experts fabricating 5-10 arrays in approximately 3-6 weeks.Improvements in fabrication throughput will come from parallelizing fabrication resources,both manpower and equipment, and by developing modest changes to fabrication techniquesand logistics. The primary requirement for increasing TES production is access to micro-fabrication resources with a particular need for dedicated thin film deposition systems toguarantee cleanliness and control of exotic materials.

• Materials Optimization and Quality Assurance Detectors for CMB-S4 will not be identi-cal. Small variations in device parameters are required to accommodate different operatingconditions associated with different observing bands, sites and instrument throughput. It isalso possible that different RF coupling schemes will be employed to optimize use of dif-ferent platforms. An important R&D goal is to identify the best materials and processingto accommodate these minor variations in TES designs such as optimal operation tempera-tures (100 vs 300 mK) and different RF couplings (see also RF coupling). This R&D shouldproceed in parallel with a program focused on understanding the connection between varia-tions in fabrication processing and superconducting RF circuit performance and mechanicalthermal properties. In addition to materials and process optimization, it is important to estab-lish test facilities and a quality assurance program among the universities, national labs andfabrication facilities that is commensurate with the increased fabrication throughput. Theultimate goal of this R&D would be an end-to-end production line yielding TES arrays withuniform properties across each wafer and consistent performance from wafer-to-wafer.

• Multiplexed TES Readout Multiplexed TES readouts are required for implementing focalplanes with more than 1000 detector elements and will continue to be an active componentfor R&D. Current multiplexer technologies already enable operation of arrays of O(10,000)detectors. Continued improvement of these multiplexing schemes will further extend thesecapabilities and recent developments of new readout techniques may lead to new multiplexertechnologies with broader applicability and lower cost.

6

3 Microwave Kinetic Inductance Detectors (MKIDs)

3.1 Description of the TechnologyMicrowave kinetic inductance detectors (MKIDs) are superconducting thin-film, GHz resonatorsthat are designed to also be optimal photon absorbers [14]. Absorbed photons with energies greaterthan the superconducting gap (ν > 2∆/h ∼= 74 GHz×(Tc/1 K)) break Cooper pairs, changing thedensity of quasiparticles in the device. The quasiparticle density affects the dissipation of the su-perconducting film and the inductance from Cooper pair inertia (kinetic inductance), so a changingoptical signal will cause the resonant frequency and internal quality factor of the resonator to shift.These changes in the properties of the resonator can be detected as changes in the amplitude andphase of a probe tone that drives the resonator at its resonant frequency. This detector technologyis particularly well-suited for sub-kelvin, kilo-pixel detector arrays because each detector elementcan be dimensioned to have a unique resonant frequency, and the probe tones for hundreds to thou-sands of detectors can be carried into and out of the cryostat on a single pair of coaxial cables (seeSection 7).

The total instrument noise is the quadrature sum of the detector noise and the photon noise, andthe fundamental performance goal is to achieve a sensitivity that is dominated by the random arrivalof background photons. For an MKID, the detector noise includes contributions from three sources:generation-recombination (g-r) noise, two-level system (TLS) noise, and amplifier noise [14]. Ingeneral, g-r noise comes from the generation and recombination of quasiparticles. Under typicaloperating conditions for ground-based CMB observations, any thermal g-r noise is negligible, sothe two main noise sources are quasiparticle generation noise from photons (photon noise) andthe associated random quasiparticle recombination noise. TLS noise is produced by dielectricfluctuations due to quantum two level systems in amorphous dielectric surface layers surroundingthe MKID. The scaling of TLS noise with operating temperature, resonator geometry, and readouttone power and frequency has been extensively studied experimentally and is well described bya semi-empirical model [15]. Finally, the amplifier noise is the electronic noise of the readoutsystem, which is dominated by the cryogenic microwave low-noise amplifier.

3.2 Demonstrated PerformanceA range of MKID-based instruments have already shown that MKIDs work at millimeter and sub-millimeter wavelengths. Early MKIDs used antenna coupling [16], and these antenna-coupledMKIDs were demonstrated at the Caltech Submillimeter Observatory (CSO) in 2007 [17] leadingto the development of MUSIC, a multi-chroic antenna-coupled MKID camera [18]. A simplerdevice design that uses the inductor in a single-layer LC resonator to directly absorb the millime-ter and sub-millimeter-wave radiation was published in 2008 [19]. This style of MKID, calledthe lumped-element kinetic inductance detector (LEKID), was first demonstrated in 2011 in the224-pixel NIKA dual-band millimeter-wave camera on the 30 m IRAM telescope in Spain [20].This pathfinder NIKA instrument led to an upgraded polarization-sensitve NIKA2 receiver withapproximately 3300 detectors [21, 22]. A large format sub-millimeter wavelength camera, calledA-MKID, with more than 20,000 pixels and a readout multiplexing factor greater than 1,000 has

7

been built and is currently being commissioned at the APEX telescope in the Atacama Desert inChile [23].

Photon noise limited horn-coupled LEKIDs sensitive to 1.2 THz were recently demonstrated [24]and these detectors will be used in the balloon-borne experiment BLAST-TNG [25, 26]. Labora-tory studies have shown that state-of-the-art MKID and LEKID designs can achieve photon noiselimited performance [27, 28, 29, 30]. And finally, MKID-based, on-chip spectrometers for sub-millimeter wavelengths (SuperSpec and Micro-Spec) are currently being developed [31, 32].

Two scalable varieties of MKID – using two completely different RF coupling strategies –are currently being developed for CMB polarization studies with CMB-S4 in mind: (i) dual-polarization lumped-element kinetic inductance detectors (LEKIDs), which are shown in Figure 2and (ii) multi-chroic MKIDs, which are shown in Figure 3 [33, 34]. The details of the RF cou-pling designs are discussed in Section ??. The horn-coupled, multi-chroic devices are based onthe polarimeters that were developed for the Advanced ACTPol experiment [35, 36], though inthe new MKID-based version, the TES bolometers are replaced with hybrid co-planar waveguide(CPW) MKIDs, and the millimeter-wave circuit is fully re-optimized for silicon-on-insulator (SOI)wafers. The multi-chroic MKIDs are still in the development stage, and a laboratory performancedemonstration will be completed in late 2016 or early 2017. The noise-equivalent temperature(NET), noise-equivalent power (NEP), in-band spectral response, pulse response (time constant),low-frequency noise performance, and multiplexing performance of LEKIDs have all been studiedextensively in the laboratory [34, 27, 28]. These studies have revealed that the performance ofLEKIDs can be compared with that of state-of-the-art TES bolometers – especially for ground-based experiments when the optical loading is greater than approximately 1 pW.

Development work is underway to make the sensing element in various MKID architectures outof materials with a tunable transition temperature, such as aluminum manganese (AlMn), titaniumnitride (TiN), TiN trilayers, and aluminum-titanium bi-layers [37, 38, ?]. With these materials it ispossible to decrease the transition temperature below that of thin-film aluminum in a controllableway, which does two critical things. First and foremost, near 150 GHz photons are energeticenough to break multiple Cooper pairs in the sensing element, so the detector noise will be furthersuppressed below the photon noise improving the sensitivity. Second, a lower Tc makes the detectortechnology sensitive to lower frequencies (∼30 GHz), so one MKID architecture with a tunabletransition temperature could be used for all of the spectral bands in CMB-S4.

3.3 Prospects and R&D Path for CMB-S4 for MKIDsMKIDs are a new detector option for CMB studies, and they may have appreciable advantagesworth considering for CMB-S4. For example, the technology was invented with high multiplexingfactors in mind, the readout uses low-power commercially available hardware, some device archi-tectures can be made from a single superconducting film, and high-performance prototype LEKIDshave been fabricated in small commercial foundries. Therefore, although MKIDs lack the heritageof TES bolometers in the CMB community, it is reasonable to anticipate that the technology couldflourish in a large-scale program like CMB-S4. To make MKIDs a viable candidate for CMB-S4instruments, research and development work must be done in the following areas:

8

polarization 1

polarization 2

capa

citiv

ely

coup

led

bias

sig

nal horn exit

aperture

cylindricalwaveguide

conicalor proled

horn

4.8 mm aperture

160 μm Si

inductors choke

IDC IDCbias

aluminumdetectorpackage

IDC

IDC

inductors

backshort prototype module (20 horns)

lterattachment

probe tones in

horn array

probe tones out to LNA

Figure 2: Left: Schematic of a dual-polarization lumped-element kinetic inductance detector(LEKID) that is sensitive to one band spectral centered on 150 GHz [34]. The LC resonatorsensitive to the horizontal polarization is colored red, while the resonator sensitive to the orthogonalpolarization is colored blue. The inductor in the resonator is the photon absorber. The dotted circlerepresents the waveguide exit aperture at the back of the horn. The resonators are driven by a probetone capacitively coupled to a transmission line for read out, which is colored green. Center: Across-sectional view of a single array element. The LEKIDs are fabricated on silicon and directlyilluminated. The horn aperture tapers to a cylindrical waveguide which also acts as a high-passfilter. A choke matches the impedance between the waveguide and the LEKID absorber, whilealso controlling lateral radiation loss along the array inside the detector module. The aluminumbottom of the module acts as the backshort, and the backshort distance is set by the silicon waferthickness. Right: A photograph of a 20-element dual-polarization LEKID module.

• Build Deployment-Quality Arrays: To date, in the spectral bands for CMB-S4, only com-paratively small arrays and scalable prototype arrays of MKIDs have been built. Theseexisting technologies will need to be scaled up and optimized for performance, yield andmanufacturability.

• Demonstrate MKIDs on the Sky: An on-sky test demonstrating that MKIDs can be usedfor high-precision CMB polarimetry is the critical next step. NIKA2 is starting to makepolarization measurements now, and this work will be informative. LEKID-based CMBpolarimeter concepts have been considered but not yet funded or built [40, 41, 38]. Dual-polarization LEKID arrays [34] with approximately 500 single-polarization detectors arecurrently being fabricated, and a demonstration using this array could take place in the nextyear or two.

• Scale-Up Fabrication Capabilities: MKIDs can be fabricated using the tools and tech-niques currently available in the foundries in national laboratories. However, the number ofdetectors required for CMB-S4 is unprecedented, so improvements in fabrication throughputwill be required. A coordinated effort among existing foundries will likely be needed.

9

hybridCPWMKID

slotline

aluminumsection

niobiumsection

microstrip from

λ/4 CPW resonator probe tones

niobiumground plane

hybridtee

band-passlters

OMT

microstrip-to-CPW coupler

hybrid tee

Figure 3: Left: One polarization sensitive multi-chroic MKID array element. Each array elementis sensitive to two polarizations and two spectral bands, so there are four MKIDs per element.Right: A schematic of the microstrip-to-CPW coupling schematic. The millimeter-wave poweris coupled from the microstrip output of the hybrid tee to the CPW of the MKID using a novel,broadband circuit [39].

• Develop Readout Software: The readout system for MKIDs requires hardware to generateand demodulate hundreds of individual microwave tones, a cryogenic low noise amplifier(LNA), and low-loss cryogenic microwave transmission lines. A well-established exampleuses the open-source ROACH FPGA hardware with associated ADC/DAC boards, a SiGeamplifier, and superconducting coaxial cables (see Figure 9). For this example construction,all of these elements are already commercially available, so the technical readiness level ishigh. For CMB-S4 any readout development work would likely focus on FPGA program-ming. Open-source software packages are already available as starting points for this work.

10

4 Time Domain Multiplexing

4.1 Technical descriptionIn time-division multiplexing (TDM), a column of detectors is combined by addressing each de-tector one at a time in a sequence. In the latest generation of the system architecture developedat NIST [42, 43], the current signal from each TES is amplified by a dedicated first-stage SQUID(actually a small series SQUID array, or SSA). Each first-stage SQUID is wired in parallel witha Josephson junction switch, and the series voltage sum of all such units in the column is ampli-fied by an SSA for transmission to the warm electronics. During multiplexing all but one of theswitches are closed to short out the inactive SQUIDs, so that only a single first-stage SQUID feedsthe SSA at any given time. This arrangement is shown schematically in Figure 4. This TDM ar-chitecture was first deployed by BICEP3 in 2015 [44], and differs substantially from the previousarchitecture [43] used in instruments such as SCUBA-2, BICEP2, and ACT.

The first-stage SQUIDs and flux-activated switches for 11 rows of a single readout columnare patterned on a single “multiplexer chip”. Each multiplexer chip is mated to a corresponding“interface chip”, which contains the parallel (shunt) bias resistors for each TES and series inductorsto define the TES bandwidth. The lines connecting the multiplexer and Nyquist chips to the TESsmust have low parasitic resistance (typically superconducting), so these chips are typically operatedat the detector temperature (0.1-0.3 K).

The Multi-Channel Electronics (MCE), developed at UBC for SCUBA-2 [45], provide biascurrents and flux offsets for all SQUID stages and switches, thus controlling the shapes and relativealignments of the various modulation curves. The warm electronics linearize this complex responsefunction through flux feedback to the first-stage SQUIDs, keeping each locked at an appropriatepoint along its modulation curve. This feedback constitutes the recorded signal for each detector.A single MCE crate contains all of the low-noise DACs, ADCs, and digital processing necessaryto operate a full TDM array of up to 32 columns and 64 rows. Each circuit board in the crate iscontrolled by an FPGA, allowing for feature additions and bug fixes through firmware updates.The entire crate communicates with a control computer through a single fiber optic pair, ensuringelectrical isolation. Multiple MCEs can be synchronized with one another (and with externalhardware) using a shared “sync box”, which distributes trigger signals and time stamps from acrystal oscillator.

Like all systems in which the multiplexing operation is carried out outside the detector wafer,the NIST TDM system makes heavy hybridization demands. We must typically make at leasteight superconducting wire bonds per TES: two each from detector to circuit board, from circuitboard to multiplexer chip, and between multiplexer and Nyquist chips, plus two for the row selectlines. Connections between the multiplexer chips and SSAs can be made with superconducting Nbwiring for low parasitic resistance and acceptable thermal isolation.

In the TDM system the number of wires to ambient temperature scales roughly as the perimeterof the 2D readout array, while the pixel count scales as the area. It requires one pair per row (rowselect) and four pair per column (bias and feedback for the first-stage SQUIDs and SSA). Theseconnections are typically twisted pairs with few-MHz bandwidth.

11

SQ

1FB

SS

A F

B

SS

A IN

SS

A B

IAS

x33

x33

x33

xN

1.0 Ω

SQ1 FB+-SQ1 BIAS+-

Series SQUID Array (SSA)

Warm Electronics

Multiplexer chip

Voltage-summingMUX chip

simplified schematic11-channels/chip

SQ1 flux feedbackfrom warm electronics

Flux-activated switch (FAS)

Further chipsin series

...

Additionalcolumns

Additionalcolumns

Additionalcolumns

Irs0

ono

time

I rs(t)

Irs1

ono

Irs1

ono

Figure 4: Schematic illustration of the voltage-summing NIST SQUID multiplexer system. EachTES is coupled inductively to a first-stage SQUID array (SQ1). All SQ1s in a column are wired inseries to the input of a series SQUID array (SSA), but at any given time all but one row of SQ1sis bypassed by a flux-activated switch. The various row select lines are biased in sequence withlow-duty-cycle square waves, as shown at left.

4.2 Demonstrated PerformanceThe TDM architecture described above is now very mature and has extensive field heritage ona variety of CMB instruments, including ABS [46], ACT [47], ACTpol [48], BICEP2 [49], BI-CEP3 [50], CLASS [51], Keck Array [52], and SPIDER [53].

The achievable multiplexing factor is constrained by the ratio of readout bandwidth to TESbandwidth. TES bandwidth is generally bounded below at several kHz by considerations of sta-bility [54]. Readout bandwidth is typically defined by the SQUID amplifier and interconnects,notably by the L/R time constant of the first-stage SQUIDs driving the SSA input coil and (insome cases) the RC time constant of the cables to ambient temperature. Advanced ACTpol is cur-rently deploying the highest achieved multiplexing factor of 64 TES channels per readout columnusing the NIST TDM chips and the UBC electronics [55].

Since the readout chain’s bandwidth must be much higher than the sampling rate of any given

12

TES, noise from the SQUIDs and warm amplifiers is heavily aliased. The aliasing penalty for r.m.s.noise is proportional to the square root of the multiplexing factor. There is some freedom to limitthe aliasing impact by reducing detector resistance or adding turns to the SQUID input coil, so inpractice the impact from the SQUID/amplifier alone has been small: BICEP2 with a 25 kHz TDMrevisit frequency experienced ∼14% aliased noise penalty to its total (photon-noise-dominated)NET, mostly from aliased detector noise [56]

Current instruments dissipate ∼1.8 nW per readout column at the detector temperature (100-300 mK) [57, 58]. This should not scale strongly with multiplexing factor, since it is dominatedby the single first-stage SQUID that is operational at any given time. The series SQUID arraysdissipate substantially more power: ∼1 µW per readout column. This power may be dissipated ata somewhat higher temperature (typically 1–4 K), and so is typically not a limiting factor.

TDM has several known crosstalk mechanisms, generally of modest amplitude [43, 49]. Thelargest form of crosstalk is inductive: each first-stage SQUID detects current from neighboringinput coils (adjacent rows in the same readout column) inductively at the ∼0.3 % level, and at ayet smaller level to more distant rows. In a well designed system, all other forms of crosstalk aresubdominant.

A typical full-sized (72-HP) MCE crate serving a ∼2000 pixel (32 column by 64 row) arrayconsumes 85 watts, supplied by custom linear or switched DC supplies. The crate dimensions areapproximately 40 × 43 × 34 cm (depth / width / height) and it weighs approximately 13 kg, notincluding separate DC supplies.

4.3 Prospects and R&D path for CMB-S4 for TDMTDM benefits from almost a decade of field experience in CMB instruments, which has yieldeddozens of publications involving more than 10,000 detectors. The hardware and software are well-characterized and well-supported. Systematics are controlled and understood for multiplexingfactors as high as 64. The interconnect technologies are also relatively simple: twisted-pair cryo-genic cables and aluminum wire bonds. Despite these successes, there are substantial developmentchallenges to scaling this technology to the high pixel counts envisioned for CMB-S4:

• Warm and Cold Interconnects Since TDM row-switching is carried out at ambient temper-ature, wires to room temperature are required for each row as well as each column. That leadsto a relatively high wire count per pixel: roughly 264 wire pairs to sub-Kelvin for a 32×200array. This may be ameliorated somewhat through the development of a custom cold switch-ing system [59]. The standard TDM system also has no provision for individually-tuned TESbias values down a common line. Larger multiplexing factors thus make heavier demands onTES fabrication uniformity (in order to use a common bias), or demand additional TES biaslines. Cold hybridization requirements are also substantial: at least eight bonds per TES,plus four per column for SQUID and TES biasing. This hybridization effort may be reducedwith fully-automated wire bonding systems or development toward indium bump-bondedsystems (e.g. [60]). The number of interconnects could be drastically reduced by fabricatingthe SQUIDs alongside the TESs on the same wafer, though this would require developmenteffort to ensure adequate uniformity and yield.

13

• Cold Electronics Fabrication The manufacture of large, high-quality Josephson junctionarrays is relatively complex, demanding careful control of superconducting film deposition.Such arrays are now manufactured routinely at e.g. NIST, but are rare in industrial fabrica-tion.

• Increased Multiplexing Factor The large number of detectors per telescope envisioned forCMB-S4, particularly for the higher-frequency instruments, will demand a higher multiplex-ing factor than has been demonstrated thus far. Careful tuning of TES and SQUID proper-ties could potentially double readout bandwidth over Advanced ACTpol while halving TESbandwidth, for a total multiplexing factor of order ∼200. Larger factors seem difficult toreconcile with current interconnect bandwidth and TES stability. A modified version of theTDM system known as “code-division multiplexing”, now under development, may proveto be more viable for larger multiplexing factors [61, 62, 63]. Rather than switching amongindividual detectors, a CDM system switches among measurements of various Walsh codecombinations (alternating-sign sums) of the various TES signals. In this configuration allTES signals are sampled at all times, eliminating the ∼

√Nmux amplifier noise aliasing

penalty. This allows for much more efficient use of readout bandwidth and thus higher mul-tiplexing factors.

Extrapolating from current technology, each 32×200 (6400 TES) readout array would incorporatemore than 70,000 Josephson junctions, 50,000 wire bonds, and ∼60 nW of power dissipation atbase temperature.

TDM brings excellent performance and extensive field experience to CMB-S4, but the chal-lenges above dampen its prospects as the sole solution for the program. BICEP3 and AdvancedACTPol have successfully deployed TDM at the ∼2000-detector scale, comparable to the pixelcounts targeted for CMB-S4’s lower frequency (e.g. 30 – 40 GHz) channels. TDM’s challengesare more daunting at the > 104-detector scale envisioned for the higher frequency receivers. TDMis nonetheless a natural back-up alternative to more ambitious multiplexing schemes.

14

5 Frequency Domain Multiplexing

5.1 Technical descriptionFrequency-domain multiplexing (FDM) takes advantage of the relatively large bandwidth of theSQUID amplifier (1 - 100 MHz) compared to the small bandwidth of CMB signal incident on aTES bolometer. Each detector is given a channel in frequency space, defined by a resonant seriesRLC circuit, with the bolometer RTES acting as a variable resistor. Each detector is ac biasedwith a unique sinusoidal carrier at its resonant frequency. Sky signals modulate RTES , whichcauses amplitude modulation in the carrier current, encoding the signals as sidebands of the carrierfrequency. A key feature of this strategy is that the bias power provided to each detector can bechosen independently, allowing the readout system to compensate somewhat for non-uniformitiesamongst detector parameters.

A circuit diagram of the FDM readout system is shown in Figure 5. A bias resistor is inparallel with the bolometer LCR circuit, with Rbias << RTES , creating a voltage bias Vbias on thebolometers. In the current system, this bias resistor is located at 4 Kelvin so that the voltage biascan be supplied by a single pair of wires to the subKelvin focal plane for each comb of bolometers.A current-biased series array of dc SQUIDs (referred to here as a “SQUID”) is used to read outa comb of n channels. The current from the bolometers is summed at the SQUID, which has amodulated output dependent on the current through the bolometers. Negative feedback is requiredto linearize the SQUID response and provide a large dynamic range. The voltage bias input mustbe nulled by sending in its inverse, to cancel out its contribution to current through the SQUID.The magnitude of sky signals must also stay within the linear regime of the SQUID. The currentgeneration of FDM uses a form of baseband feedback known as Digital Active Nulling (DAN)[64], where feedback is applied only around the bolometer carrier frequencies. With DAN, the skysignal is also nulled, so that the SQUID acts as an error sensor, and the nulling current is the skysignal.

The readout bandwidth is set by the inductance L and the resistance RTES . The inductanceof each channel is constant, and the capacitance is varied to set the resonant frequency. In cur-rent implementations of the FDM system, there is a comfortable margin between the necessaryoptical time constant, the detector time constant, and the readout time constant (L = 60µH ,τe ∼ 0.5ms)[?]. The spacing of the channels in a frequency comb must be large enough so thatthe off-resonance current from neighboring channels does not spoil the voltage bias on-resonance,and so that crosstalk between neighboring channels is small. In the current system, the resonantfrequencies range from 1 - 6 MHz, with a minimum spacing of 40 KHz to keep crosstalk below0.5%.

The operation of the resonant RLC circuit depends on there being negligible impedance inseries with the well-defined components of the circuit. The bolometer resistance RTES must bethe dominant resistance, and there also must be minimal stray inductance from wiring and cir-cuit boards. These components and wiring are all at sub-Kelvin temperatures, which helps toachieve these specifications. The wiring from the SQUID and bias resistor at 4 Kelvin to the sub-Kelvin focal plane must be low inductance while acting as a thermal break, which is achieved withbroadside-coupled NbTi striplines with lengths ∼ 50 − 100cm. For systems where the bias resis-

15

!

"# $%

&

'"(

)*+'"(

Figure 5: A circuit diagram of the frequency-domain multiplexing readout system is shown, withthe cryogenic portion at the left, and the room temperature electronics at right. Figure from [?]

tor and SQUID input sit at different cryogenic temperatures, the practical lengths and inductancesof the current sub-Kelvin wiring and components requires RTES ≈ 1Ω, to keep the bolometerimpedance large compared to other impedances. FDM for much lower RTES has been imple-mented by placing the bias resistor and a first stage SQUID at low temperature, such that the wirelengths are short and wiring inductance is negligible [?].

There are 2 pairs of wirebonds per detector: from detector wafer to cable, and across the LCresonator. There is no power dissipation at the subKelvin stages from readout components. Thecryogenic wiring is simple: there is just one pair of wires running to the subKelvin stages for eachmultiplexed comb of bolometers. There are only two connectors used in the readout chain so as tominimize stray inductance and resistance: after the wafer readout cable, and at the 4K SQUID. Thethermal load on the focal plane from these wires is about ∼ 1nW per comb, depending on wirelength and the number of thermal interfaces. There is also minimal power dissipated at the othertemperature stages from readout wiring, relative to other contributions and the cooling capacity.The readout system dissipates power at the 4 Kelvin stage in the current system, from SQUIDs andbias resistors,<≈ 1µW per comb. If the bias resistor is moved to a cold stage, it can be replacedwith an inductive or capacitive divider to reduce dissipation to zero. The standard twisted-paircryogenic wiring which runs between the cold components and the room temperature electronicsrequires 3 pairs for each multiplexed comb, and can be long in length.

The FDM readout system uses custom warm electronics designed at McGill University [65],[64], [66]. These synthesize the bolometer voltage biases (labeled “Carrier Bias Comb”), thenulling signal that is applied to the SQUID to increase its dynamic range (labeled “Nulling Comb”),and the demodulators. The SQUID has a transimpedance that is high enough to convert smallcurrent through the bolometers into a voltage that is read out with a room-temperature amplifier.The power and space requirements for the warm electronics are relatively small. The “ICE” roomtemperature readout electronics [?] being deployed for SPT3g with a 68x multiplexing factor willoperate 8,700 detector channels per 9U crate (40cm tall, 25cm deep, 50cm wide), with less than 1kW of power draw.

16

5.2 Demonstrated PerformanceStage-2 CMB polarization experiments have demonstrated frequency-domain multiplexing factorsof 8× - 16× on a single pair of cryogenic wires, including POLARBEAR-1, [?, ?], SPT-Pol[?, 67],and EBEX[68]. The Stage-3 experiments SPT-3G[?] and POLARBEAR-2[?, ?, ?] are deployingin 2016 and 2017 with multiplexing factors of 68× and 40×, respectively.

The usable bandwidth was extended by changing to a form of baseband feedback known asDigital Active Nulling (DAN) [64], where feedback is applied only around the bolometer carrierfrequencies. Channels can be placed anywhere in the SQUID bandwidth, which for the seriesarrays presently used is 120 MHz. DAN feedback has been demonstrated on the sky with theSPT-Pol experiment. To improve the precision of channel placement and to reduce loss at higherfrequencies, superconducting resonator components were developed [?, ?], with an interdigitatedcapacitor along with a spiral inductor in a single layer of superconducting traces.

These developments greatly increased the potential multiplexing factor for the FDM systemused in Stage-3 experiments.

The system can be designed so that the dominant readout noise sources contribute less than15% of the minimum expected noise equivalent current caused by the bolometer power noiseterms (∼ 20 − 30pA

√Hz), and is negligible compared to the photon noise for detectors with

appropriate parameters. Two of the dominant noise sources are related to the SQUID: the currentnoise of the SQUID itself, and the voltage noise of the SQUID’s first-stage amplifier. Both of thesenoise sources are far from fundamental limits and could be further reduced. There is also noiseassociated with the generation of the carrier and nuller signals, dominated by the output currentnoise of the digital-to-analog converter (DAC), presently limited by available off-the-shelf DACtechnology.

In the FDM system, signal crosstalk onto a detector can only occur if the crosstalk signal lieswithin its frequency bandwidth. Other crosstalk can introduce excess loading on the SQUID andnoise, but does not introduce false sky signals. Since the fraction of total SQUID bandwith thatis occupied by detector signals is very small, there is a small amount of crosstalk from its nearestneighbors in frequency space and physical space in the comb.

POLARBEAR-1 had the highest level of signal crosstalk from neighbors in frequency space,with a maximum level of about 1%[?]. Stage-3 experiments expect similar low levels of crosstalk,but this needs to be demonstrated on the sky for the larger multiplexing factors used.

5.3 Prospects and R&D path for CMB-S4 for FDM• Scaling up to S4 with current technology Scaling up the current system, without any im-

provements to the cold or warm readout electronics and maintaining the present multiplexingfactor of 68 per wire-pair, would mean that the number of readout modules needed for a sin-gle telescope of 50,000 detectors would increase by a factor of ∼ 4 compared to Stage 3experiments (SPT-3G has 15,234 detectors at 68× multiplexing). The “ICE” readout boardsthat are used for FDM are also used for radio astronomy correlators [?]. For example, theCHIME telescope is deploying with 128 readout boards (4.5 times more than SPT3g) in2016, demonstrating that systems with this number of boards is tractable.

17

Scaling to cryogenic FDM circuits up is more challenging. One potential issue is that itcould require long lengths of wiring to the bolometers (compared to the current lengths of∼ 50 − 100 cm). The thermal loads on the subKelvin stages from ∼ 1500 readout wireswould require a significant portion of the cooling capacity of a three-stage helium sorptionfridge. In a dilution refrigerator cooled cryostat, the SQUID and bias resistor could be movedto the 1 Kelvin buffer stage if it could accomodate the ∼ 1mW of power dissipated, alongwith the thermal load from 3 pairs of wires per comb. This would greatly reduce the physicaldistance and necessary wiring lengths to the bolometers.

• Prospects for increasing multiplexing factor The issues associated with the cryogenicwiring complexity would be addressed by increasing the multiplexing factor up to 128-256×. This can either be achieve by packing the channels closer together, with narrowerinductor-capacitor resonances, or by using more bandwidth to accommodate additional fre-quency channels. Presently, the latter strategy is an active path of development. Extendingthe present system in this way requires high uniformity in the channel spacing, and excellentcontrol of stray impedances in the wiring and interconnects.

An alternate strategy is to reduce strays by keeping all components of the cold-multiplexerat sub-kelvin temperature, ensuring short wire lengths. High frequency superconductingresonators were integrated on detector wafers as shown in Figure 6. High frequency read-out shrinks physical size of resonators to facilitate integration and widens available read-out bandwidth for higher multiplexing count. Integration of frequency multiplxing circuitand detector simplifies interconnects by reducing number of required connection by multi-plexing factor. Also parasitic impedance can be accurately simulated and controlled withmicro-fabrication technique used in detector RF circuit design. Integrated frequency multi-plexing circuit can be coupled with microwave SQUID and high frequency DfMUX readout.Currently 50–100 MHz resonators are being developed for frequency domain multiplexingreadout. Same method can be used to fabricate ∼1 GHz resonators for microwave SQUIDsreadout which would be significantly smaller and take less space on a detector wafer.

Figure 6: Photograph of superconducting resonators fabricated on same wafer as multi-chroicdetector

• Warm readout electronics capabilities Increasing the backend electronics multiplexingfactor is not an issue. Firmware for the “ICE” backend electronics already supports a mul-

18

tiplexing factor of 128x and uses about half the FPGA resources. Exploiting full FPGAresources and minor optimizations should allow an increase to 256x without warm hardwarechanges (∼32,000 detector channels for a single 9U crate). The ICE backend electronicsused for TES FDM could also be specialized for higher frequency (100 MHz or 1 GHz)FDM readout of KIDs or mSQUIDs by using higher frequency digitizer daughter-boardsthat are available commercially, or by developing custom daughter boards. For telescopesthat plan to support deployment of both TES and KID focal planes, a system that can supportFDM at high and low frequencies would allow the same core electronics to be used for thereadout of both detector systems.

19

0

♦

5.45 5.50 5.55 5.60 5.6520

15

10

5

0

Tra

nsm

issio

n (

dB

)

Frequency (GHz)

st

t①

②

s s

|S2

1|2

(dB

)

Frequency (GHz)5.454 5.456 5.458 5.46

-10

-20

st

Figure 7: Overview of the the microwave SQUID multiplexer. (a) Schematic of the circuit. (b) Photograph of a 32-channel µMUX chip. (c) S21 transmission measurement of the µMUX with 32 active channels. (d) Variation of singlereadout channel transmission curves to applied input magnetic flux (or equivalently applied current when inductivelycoupled).

6 Microwave SQUIDs

6.1 Technical descriptionThe microwave SQUID multiplexer (µMUX) [69, 70] is a readout scheme intended to greatly in-crease the focal plane pixel count of Transition Edge Sensor (TES) bolometer arrays. The tech-nology is inspired by the GHz frequency domain multiplexing approach of microwave kineticinductance detectors (MKIDs) [71, 72]. Fig. 7 illustrates the concept. Current sourced from a TEStransduces a frequency shift in a microwave resonator by means of a flux-coupled RF SQUID. TheTES bias circuit is identical to that of time-division SQUID multiplexing (TDM). The TES is DCbiased by means of applying a constant current to a shunt resistor that is wired in parallel with theTES. Many detectors may be biased using a single DC current source and two wires. The readoutmechanism, however, is very different than TDM. Each TES is inductively coupled to it’s own res-onator at a unique resonance frequency. In this manner, many readout channels densely pack onto asingle superconducting transmission line with a total readout bandwidth of several GHz. Similar toMKID readout, signals are determined from the transmission properties of microwave resonators,monitored by use of homodyne readout techniques. The only difference of µMUX readout, withrespect to MKID readout, is the addition of ‘flux-ramp demodulation’ [73], which linearizes theresponse and substantially decreases 1/f readout noise. Cabling per module consists of a pair ofDC wires and two coaxial cables.

6.2 Demonstrated PerformancePerformance has been demonstrated through extensive lab-based measurements and with on-skyobservations in the MUSTANG2 receiver. Readout noise levels relevant for CMB-S4 have already

20

been demonstrated in the lab. The architecture was used to read out a 3×10−17 W/√

Hz noiseequivalent power (NEP) TES bolometer, which was optimized for CMB polarization measure-ments [70]. In this demonstration, the readout noise was negligible compared to the system noise,to modulation frequencies as low as 1 Hz. Lower modulation frequencies were not investigated.By altering the input coil coupling, the current generation of µMUX chips have achieved an input-referred white current noise level of 17 pA/

√Hz [74]. This noise level is nearly a factor of 10

below the expected photon noise level in the types of cryogenic receivers envisioned for CMB-S4. (Here we assume 3 pWs of optical load at 150 GHz, and an optimized TES bolometer withRTES ∼ 5 mΩ.)

On-sky observations have been made in two engineering runs of the MUSTANG2 receiver onthe Green Bank Telescope (GBT). In 2015, MUSTANG2 used the architecture in a 32-channelper module configuration to make first-light images [75]. In 2016, on-sky, background-limitedsensitivity has been demonstrated in pixels coupled to 64-channel multiplexers.

In addition to bolometric applications, the µMUX is under development for several TES microcalorimeter-based instruments. A DOE-funded, 512-pixel gamma-ray spectrometer demonstration, calledsledgehammer, is underway [74], and the readout approach is baselined for a first-light instru-ment at the Linac Coherent Light Source II (LCLS-II). The current technical state-of-the-art forcalorimetric applications is a successful demonstration of undegraded energy resolution in a 4-pixelarray that was read out using the scalable ROACH-II warm electronics.

The total readout bandwidth and resonator frequency spacing set the number of detectors mul-tiplexed on a single coaxial cable. To date, the µMUX development has focused on the 4-8 GHzband since this matches the bandwidth of existing cryogenic HEMT amplifiers. Resonator spac-ing of 6 MHz has been demonstrated with few resonator collisions. Hence in the implementedapproach and using the full HEMT bandwidth, 660 detectors can be multiplexed on a single line.

Multiple generations of 33-channel multiplexers using a standard 3mm × 19mm form factorhave been fabricated and tested. Different frequency band 33-channel chips have been wired inseries to increase the number of readout channels per multiplexer module. This demonstratesboth successful frequency scaling of the devices and the ability to daisy chain chips together, asa means to increase the multiplexing factor. The silicon footprint is currently identical to that oftime-division SQUID multiplexing.

The total power dissipated on the cold stage is ∼ 10 pW/channel. When resonators are spacedin frequency at least ten times their bandwidth, nearest neighbor crosstalk is measured to be< 0.1 %. Linearity has been measured to 1 part in 1,000.

6.3 Prospects and R&D path for CMB-S4 for microwave SQUIDsThe µMUX is less mature than time-division SQUID multiplexing (TDM) or MHz frequency-division SQUID multiplexing (FDM), which together have been used to readout ∼30,000 de-ployed TES detectors that observe in the millimeter/sub-millimeter/FIR. However, the technologyis rapidly gaining maturity through its use in several instruments. The demonstrated multiplex-ing factor is equal to that of contemporary TDM instruments (x64, Advanced ACTPol [76]) andFDM instruments (x68, SPT-3G [77]). The envisioned multiplexing factor is at least an order ofmagnitude higher than this.

21

R&D items for CMB-S4 include:

• Multiplexing Density With current technology, the cold multiplexing density achieves amultiplexing factor of 660. Recent developments in fabrication have reduced the frequencyscatter by several factors, and thus the multiplexing density may be increased by this samefactor. Near-term efforts to demonstrate∼ 1 MHz frequency spacing would be beneficial, asthe quantity of warm readout electronics boards and cryogenic HEMT amplifiers reduces bythis same factor.

• Array Performance Demonstration The majority of experimental data on the µMUX is atthe few pixel demonstration level. On-sky results from MUSTANG2 are promising (whichuses a ×64 multiplexing factor), but a detailed study of performance, including low fre-quency noise properties, cross-talk and linearity is needed. MUSTANG2 offers a nice plat-form for this study, but other instruments or lab-based work will be essential since MUS-TANG2 will not probe the target ∼ 1 MHz frequency spacing.

• Smaller Circuit Elements Smaller circuit elements will reduce the cost of any cryogenicreadout technology, since fewer wafer need to be processed for a fixed number of readoutchannels. The footprint of the circuitry is currently comparable or several factors smallerthan the leading TES multiplexing approaches. By moving to a lumped element design, thereadout footprint could shrink substantially.

• Integrated Detector Fabrication The current µMUX implementation decreases the numberof wires running between temperature stages. However, four to six wirebonds per channelare still required at the cold (isothermal) focal plane stage. Placing the readout onto thedetector wafer solves this issue, drastically reduces the complexity of focal plane assembly,and eliminates the need for separate SQUID multiplexer chip fabrication. Beam formingelements, such as lenslets or feedhorns, which create space on the wafer for the readoutcomponents make integrated fabrication a possibility. In the near term, preliminary designsshould be pursued, and steps to demonstrate an integrated fabrication process flow should betaken.

• Warm Electronics Development Warm readout for the µMUX has heavily benefitted fromthe developments in MKID readout. But future R&D is required and discussed in the fol-lowing section.

Lastly, we note that the µMUX may be developed as a stand-alone multiplexing technique forCMB-S4, or it may find use in a hybrid multiplexing scheme. Hybrid multiplexing is a commonway to more efficiently use available bandwidth. For example, it is the approach used in 3G cellphone technology. For readout of TES bolometers, a lower bandwidth multiplexing scheme, suchas TDM or code-division multiplexing, is embedded within a wider bandwidth GHz resonator.Therefore each GHz tone carries the signals from N bolometers, where N may be 32 or 64. Aproof-of-concept demonstration was shown in 2008 [78]. In the near term, a design study forhybrid multiplexing should be undertaken to inform CMB-S4, and if determined viable, an R&Dpath laid out.

22

7 Room-temperature electronics for readout in the frequencydomain

7.1 Description of the TechnologyTwo of the three readout schemes for TES detectors discussed in this paper as well as all MKIDreadout operate in the frequency domain. The response to a tone played at the fundamental resonantfrequency of an MKID or a resonator coupled to a TES is measured for amplitude and/or phaseshift. A signal in FDM is an amplitude modulation, in µMUX is a phase shift in the resonance,and in MKIDs is a shift both in phase and amplitude. In this section, we focus on the warmreadout electronics for µMUX and MKIDs, which operate in GHz frequencies and enjoy extensivecommonality in the architecture of their readout electronics. MHz FDM shares the same overallstrategy, albeit at a lower frequency. We will also discuss the possibility of the same backendelectronics supporting all three of these techniques.

Figure 8 shows the readout schematic for MKID arrays and the µMUX multiplexer. Thesesystems operate at the range of the resonance frequencies of the detectors, which is typically 100– 8,000 MHz. They are designed to support a sufficiently large bandwidth (500 – 2000 MHz)to readout hundreds or thousands of detectors at once, depending on the resonator quality factorsand frequency spacing. The readout noise is much less than the intrinsic detector noise (below∼ −90 dBc/Hz) with frequency resolution to probe resonators with very high quality factors, Q∼ 100, 000 [79, 80, 81, 82, 83, 84, 85, 86, 87, 88].

A common readout design implemented by various MKID experiments including AMKID [89],BLAST-TNG [90], MAKO [91], MUSIC [92], NIKA [93], and NIKA-2 [94] makes use of a ho-modyne readout technique. A digital tone generator, such as an FPGA, is connected to a DAC toproduce the probe tones. The waveforms are generated on the FPGA by taking a length N IFFTof a delta function comb. The length of the IFFT sets the frequency resolution of the tones. Forexample, an FPGA with 500 MHz of bandwidth spaced with N = 218 bins gives a frequency reso-lution of about 1.9 kHz. The initial waveform amplitude should be maximal within the range of theDAC and the waveform crest factor should be minimized. This is achieved by randomly generat-ing the probe tone phases (more advanced techniques are unnecessary because the MKID devicesthemselves will quasi-randomly shift the tone phases) [79]. A mixing circuit is used to bring thesignals to the required frequency. For example, to read out devices with resonances between 1000to 1500 MHz, one would use the FPGA to generate tones from -250 to 250 MHz and mix themwith a 1250 MHz local oscillator and IQ modulator to the required frequency. The same LO isused to demodulate the tones after passing through the MKID array. The tones are then fed intothe cryostat via coaxial cables and vacuum feedthroughs. The coax is then wired through to therequired cold stages and attenuated before interacting with the detectors. The signal is then passedinto a cold low noise amplifier and then back out of the cryostat. The signal is again amplifiedand mixed down before going into an ADC and back into the digital readout. Signals are thendemodulated into amplitude and phase shifts, which can be calibrated to intensity variations on thedetectors.

The warm electronics currently used for the µMUX borrows largely from MKID readout de-

23

velopments. One additional requirement for the µMUX system is a flux modulation signal, whichlinearizes the rfSQUID response and has the significant advantage of moving the signal band awayfrom resonator system 1/f noise. MUSTANG2 and NIST have enhanced the ROACH frameworkto include this capability for µMUX (cite).

7.2 Demonstrated PerformanceA combination of the digital readout bandwidth and resonator quality factors determine the maxi-mum number of detectors that can be readout on a single coaxial line. State of the art microwavereadout systems can support thousands of resonators [95, 79, 84, 88, 87, 96, 90, 83]. In lab systemshave demonstrated multiplexing factors up to 1000 while maintaining the required noise perfor-mance [79, 97]. NIKA2 has demonstrated the highest on-sky multiplexing factor of 400 [94, 86].

The readout heat loading in the cryostat is due to the RF signal and LNA power usage. The RFpower dissipated at the cold stage depends on the design of the resonators and the input power. Forthe µMUX system, this turns out to be 10 pW per channel. The LNA dissipates 5-10 milliwatts ofpower, but it is thermalized at a warmer stage (∼4 K) which has substantially more cooling power.The total power consumption of the system is dominated by the warm electronics and totals around50–100W [79, 97, 86].

The readout noise should be sub dominant to the detector noise at all frequencies, and this hasbeen demonstrated in a variety of MKID instruments [94, 86, 79, 97, 98] at sub-millimeter andmillimeter frequencies.

7.3 Prospects and R&D path for CMB-S4 for microwave readoutCurrent tone generation and multiplexing schemes are capable of reading out thousands of detec-tors on a single pair of coaxial cables. In order to meet the stability, noise, and power requirementsfor CMB detectors, the readout systems benefits from development in the following areas:

• Faster ADC/DACs Increasing the bandwidth of the digital electronics directly increases themultiplexing factor of the system.

• Low-frequency noise Minimizing the low frequency noise of the readout system is criticalfor CMB-S4. To mitigate the low-frequency noise, readout systems commonly place tonesoff resonance to characterize correlated electronic noise, which is subsequently subtractedfrom the detector data [79, 97]. Work should be done to assess if this is sufficient andimprove the LNA, ADC, and DAC stability.

• Increased linearity The total power on the LNA must be more than 20 dB below the third-order intercept. The driving power for MKIDs is -90 dBm, allowing for nearly 10,000 de-tectors on a single line. However, the µMUX system is driven at a higher power and runsinto this limit at ∼1000 detectors. An alternative warm readout design is being developedat Stanford/SLAC. It uses direct digital synthesis and demodulation to eliminate the tun-ing parameters necessary for the IQ scheme. This system also performs resonant frequencytracking, which is inherently less susceptible to low-frequency amplitude and phase noise. A

24

Lg + Lk

R

C1

Cc

Cc

Z0

Z0Z0

out

2V

LNA

Lg + Lk

R

C2

Cc

Cc

Z0 Z0 Z0

Lg + Lk

R

CN

Cc

Cc

Z0

each detectorhas uniquecapacitance

resonator

rf-SQUID

TES0loop

flux-ramp0line

0LNA

out

each0resonance0has0a0unique0length

Z0 Z0 Z0 Z0

Z0 Z0 Z0

Figure 8: Circuit schematics showing how MKIDs and µMUX are multiplexed. Left: Each MKIDhas a unique resonance frequency, which is set by the capacitor in the resonator, for example.A comb of probe tones is routed to the MKID array using a single transmission line, and singlecryogenic low-noise amplifier (LNA) is used read out all of the detectors. Right: Each µMUX hasa unique resonance frequency set by the length of the quarter-wavelength resonator. µMUX alsouses a comb of probe tones and a single LNA to read out many detectors.

major benefit of resonant frequency tracking is that it reduces the signal level to the follow-onLNA and allows more channels per output before running into linearity issues.

• Clock distribution stability Instability, drift and jitter in the clock distribution will introduce1/f noise into the system.

• Universal backend electronics The backend electronics used for TES FDM could also bespecialized for higher frequency (100 MHz or 1 GHz) FDM readout of KIDs or µMUX

by using higher frequency digitizer daughter-boards that are available commercially, or bydeveloping custom daughter boards. For telescopes that plan to support deployment of bothTES and KID focal planes, a system that can support FDM at high and low frequencieswould allow the same core electronics to be used for the readout of both detector systems.

25

DCblock

DCblock

DCblock

-20 dB attenuator

LNA

-20 dB attenuator

duroid

duroid

detector array

SScoax

SScoax

SScoax

SScoax

NbTicoax

CuNicoax

45 K 3.5 K 100 mK

Cryostat

SMAfeed-

through

Low-PassFilters

Low-PassFilters

DigitalAttenuator

DAC

ADC

ROACH-2Board

Amplier

lterbank

300 K

LocalOscillator

ROACH-2

ADC/DAC

10 Gbit Ethernet

attenuator

mixers

amplierLO

Figure 9: Top: Readout schematic showing the probe tone path. The top left shows the signalgeneration, digital-to-analog conversion, and IQ mixing. The blue portion shows the cryogenicpart of the system. The bottom left shows the demodulation and filtering scheme. Bottom Left:The ROACH-2 with the DAC/ADC. Bottom Right: The analog signal conditioning hardware.This chassis houses the filters, room-temperature mixers, attenuator, warm amplifier, and the localoscillator shown in the schematic above. Existing hardware has a multiplexing factor of approxi-mately 500 and each readout consumes only 20 W.

26

References[1] K. D. Irwin. An application of electrothermal feedback for high resolution cryogenic particle

detection. Applied Physics Letters, 66:1998–2000, April 1995.

[2] K.D. Irwin and G.C. Hilton. Transition-edge sensors. In Christian Enss, editor, CryogenicParticle Detection, volume 99 of Topics in Applied Physics, pages 63–150. Springer BerlinHeidelberg, 2005.

[3] J. Zmuidzinas. Thermal noise and correlations in photon detection. Applied Optics, 42:4989–5008, September 2003.

[4] J. C. Mather. Bolometer noise: nonequilibrium thoery. Applied Optics, 21:1125–1129, March1982.

[5] K. Arnold, P. A. R. Ade, A. E. Anthony, D. Barron, D. Boettger, J. Borrill, S. Chap-man, Y. Chinone, M. A. Dobbs, J. Errard, G. Fabbian, D. Flanigan, G. Fuller, A. Ghribi,W. Grainger, N. Halverson, M. Hasegawa, K. Hattori, M. Hazumi, W. L. Holzapfel,J. Howard, P. Hyland, A. Jaffe, B. Keating, Z. Kermish, T. Kisner, M. Le Jeune, A. T. Lee,E. Linder, M. Lungu, F. Matsuda, T. Matsumura, N. J. Miller, X. Meng, H. Morii, S. Moy-erman, M. J. Myers, H. Nishino, H. Paar, E. Quealy, C. Reichardt, P. L. Richards, C. Ross,A. Shimizu, C. Shimmin, M. Shimon, M. Sholl, P. Siritanasak, H. Spieler, N. Stebor, B. Stein-bach, R. Stompor, A. Suzuki, T. Tomaru, C. Tucker, and O. Zahn. The bolometric focal planearray of the POLARBEAR CMB experiment. In Millimeter, Submillimeter, and Far-InfraredDetectors and Instrumentation for Astronomy VI, volume 8452 of Proc. SPIE, page 84521D,September 2012.

[6] B. Reichborn-Kjennerud, A. M. Aboobaker, P. Ade, F. Aubin, C. Baccigalupi, C. Bao, J. Bor-rill, C. Cantalupo, D. Chapman, J. Didier, M. Dobbs, J. Grain, W. Grainger, S. Hanany,S. Hillbrand, J. Hubmayr, A. Jaffe, B. Johnson, T. Jones, T. Kisner, J. Klein, A. Korotkov,S. Leach, A. Lee, L. Levinson, M. Limon, K. MacDermid, T. Matsumura, X. Meng, A. Miller,M. Milligan, E. Pascale, D. Polsgrove, N. Ponthieu, K. Raach, I. Sagiv, G. Smecher, F. Stivoli,R. Stompor, H. Tran, M. Tristram, G. S. Tucker, Y. Vinokurov, A. Yadav, M. Zaldarriaga, andK. Zilic. EBEX: a balloon-borne CMB polarization experiment. In Millimeter, Submillime-ter, and Far-Infrared Detectors and Instrumentation for Astronomy V, volume 7741 of Proc.SPIE, page 77411C, July 2010.

[7] J. T. Sayre, P. Ade, K. A. Aird, J. E. Austermann, J. A. Beall, D. Becker, B. A. Benson,L. E. Bleem, J. Britton, J. E. Carlstrom, C. L. Chang, H.-M. Cho, T. M. Crawford, A. T.Crites, A. Datesman, T. de Haan, M. A. Dobbs, W. Everett, A. Ewall-Wice, E. M. George,N. W. Halverson, N. Harrington, J. W. Henning, G. C. Hilton, W. L. Holzapfel, J. Hubmayr,K. D. Irwin, M. Karfunkle, R. Keisler, J. Kennedy, A. T. Lee, E. Leitch, D. Li, M. Lueker,D. P. Marrone, J. J. McMahon, J. Mehl, S. S. Meyer, J. Montgomery, T. E. Montroy, J. Nagy,T. Natoli, J. P. Nibarger, M. D. Niemack, V. Novosad, S. Padin, C. Pryke, C. L. Reichardt, J. E.Ruhl, B. R. Saliwanchik, K. K. Schaffer, E. Shirokoff, K. Story, C. Tucker, K. Vanderlinde,

27

J. D. Vieira, G. Wang, R. Williamson, V. Yefremenko, K. W. Yoon, and E. Young. Design andcharacterization of 90 GHz feedhorn-coupled TES polarimeter pixels in the SPTPol camera.In Millimeter, Submillimeter, and Far-Infrared Detectors and Instrumentation for AstronomyVI, volume 8452 of Proc. SPIE, page 845239, September 2012.

[8] K. L. Denis, A. Ali, J. Appel, C. L. Bennett, M. P. Chang, D. T. Chuss, F. A. Colazo,N. Costen, T. Essinger-Hileman, R. Hu, T. Marriage, K. Rostem, K. U-Yen, and E. J. Wol-lack. Fabrication of Feedhorn-Coupled Transition Edge Sensor Arrays for Measurementof the Cosmic Microwave Background Polarization. Journal of Low Temperature Physics,184:668–673, August 2016.

[9] R. J. Thornton, P. A. R. Ade, S. Aiola, F. E. Angile, M. Amiri, J. A. Beall, D. T. Becker,H. Cho, S. K. Choi, P. Corlies, K. P. Coughlin, R. Datta, M. J. Devlin, S. R. Dicker, R. Dun-ner, J. W. Fowler, A. E. Fox, P. A. Gallardo, J. Gao, E. Grace, M. Halpern, M. Hasselfield,S. W. Henderson, G. C. Hilton, A. D. Hincks, S. P. Ho, J. Hubmayr, K. D. Irwin, J. Klein,B. Koopman, D. Li, T. Louis, M. Lungu, L. Maurin, J. McMahon, C. D. Munson, S. Naess,F. Nati, L. Newburgh, J. Nibarger, M. D. Niemack, P. Niraula, M. R. Nolta, L. A. Page, C. G.Pappas, A. Schillaci, B. L. Schmitt, N. Sehgal, J. L. Sievers, S. M. Simon, S. T. Staggs,C. Tucker, M. Uehara, J. van Lanen, J. T. Ward, and E. J. Wollack. The Atacama CosmologyTelescope: The polarization-sensitive ACTPol instrument. ArXiv e-prints, May 2016.

[10] J. W. Henning, P. Ade, K. A. Aird, J. E. Austermann, J. A. Beall, D. Becker, B. A. Benson,L. E. Bleem, J. Britton, J. E. Carlstrom, C. L. Chang, H.-M. Cho, T. M. Crawford, A. T.Crites, A. Datesman, T. de Haan, M. A. Dobbs, W. Everett, A. Ewall-Wice, E. M. George,N. W. Halverson, N. Harrington, G. C. Hilton, W. L. Holzapfel, J. Hubmayr, K. D. Irwin,M. Karfunkle, R. Keisler, J. Kennedy, A. T. Lee, E. Leitch, D. Li, M. Lueker, D. P. Marrone,J. J. McMahon, J. Mehl, S. S. Meyer, J. Montgomery, T. E. Montroy, J. Nagy, T. Natoli, J. P.Nibarger, M. D. Niemack, V. Novosad, S. Padin, C. Pryke, C. L. Reichardt, J. E. Ruhl, B. R.Saliwanchik, J. T. Sayre, K. K. Schaffer, E. Shirokoff, K. Story, C. Tucker, K. Vanderlinde,J. D. Vieira, G. Wang, R. Williamson, V. Yefremenko, K. W. Yoon, and E. Young. Feedhorn-coupled TES polarimeter camera modules at 150 GHz for CMB polarization measurementswith SPTpol. In Millimeter, Submillimeter, and Far-Infrared Detectors and Instrumentationfor Astronomy VI, volume 8452 of Proc. SPIE, page 84523A, September 2012.

[11] BICEP2 Collaboration, Keck Array Collaboration, SPIDER Collaboration, P. A. R. Ade,R. W. Aikin, M. Amiri, D. Barkats, S. J. Benton, C. A. Bischoff, J. J. Bock, J. A. Bonetti,J. A. Brevik, I. Buder, E. Bullock, G. Chattopadhyay, G. Davis, P. K. Day, C. D. Dowell,L. Duband, J. P. Filippini, S. Fliescher, S. R. Golwala, M. Halpern, M. Hasselfield, S. R.Hildebrandt, G. C. Hilton, V. Hristov, H. Hui, K. D. Irwin, W. C. Jones, K. S. Karkare,J. P. Kaufman, B. G. Keating, S. Kefeli, S. A. Kernasovskiy, J. M. Kovac, C. L. Kuo, H. G.LeDuc, E. M. Leitch, N. Llombart, M. Lueker, P. Mason, K. Megerian, L. Moncelsi, C. B.Netterfield, H. T. Nguyen, R. O’Brient, R. W. Ogburn, IV, A. Orlando, C. Pryke, A. S. Rahlin,C. D. Reintsema, S. Richter, M. C. Runyan, R. Schwarz, C. D. Sheehy, Z. K. Staniszewski,R. V. Sudiwala, G. P. Teply, J. E. Tolan, A. Trangsrud, R. S. Tucker, A. D. Turner, A. G.

28

Vieregg, A. Weber, D. V. Wiebe, P. Wilson, C. L. Wong, K. W. Yoon, and J. Zmuidzinas.Antenna-coupled TES Bolometers Used in BICEP2, Keck Array, and Spider. Astrophys. J.,812:176, October 2015.

[12] Z. D. Kermish, P. Ade, A. Anthony, K. Arnold, D. Barron, D. Boettger, J. Borrill, S. Chap-man, Y. Chinone, M. A. Dobbs, J. Errard, G. Fabbian, D. Flanigan, G. Fuller, A. Ghribi,W. Grainger, N. Halverson, M. Hasegawa, K. Hattori, M. Hazumi, W. L. Holzapfel,J. Howard, P. Hyland, A. Jaffe, B. Keating, T. Kisner, A. T. Lee, M. Le Jeune, E. Linder,M. Lungu, F. Matsuda, T. Matsumura, X. Meng, N. J. Miller, H. Morii, S. Moyerman, M. J.Myers, H. Nishino, H. Paar, E. Quealy, C. L. Reichardt, P. L. Richards, C. Ross, A. Shimizu,M. Shimon, C. Shimmin, M. Sholl, P. Siritanasak, H. Spieler, N. Stebor, B. Steinbach,R. Stompor, A. Suzuki, T. Tomaru, C. Tucker, and O. Zahn. The POLARBEAR experiment.In Millimeter, Submillimeter, and Far-Infrared Detectors and Instrumentation for AstronomyVI, volume 8452 of Proc. SPIE, page 84521C, September 2012.

[13] E. M. George, P. Ade, K. A. Aird, J. E. Austermann, J. A. Beall, D. Becker, A. Bender, B. A.Benson, L. E. Bleem, J. Britton, J. E. Carlstrom, C. L. Chang, H. C. Chiang, H.-M. Cho, T. M.Crawford, A. T. Crites, A. Datesman, T. de Haan, M. A. Dobbs, W. Everett, A. Ewall-Wice,N. W. Halverson, N. Harrington, J. W. Henning, G. C. Hilton, W. L. Holzapfel, S. Hoover,N. Huang, J. Hubmayr, K. D. Irwin, M. Karfunkle, R. Keisler, J. Kennedy, A. T. Lee,E. Leitch, D. Li, M. Lueker, D. P. Marrone, J. J. McMahon, J. Mehl, S. S. Meyer, J. Mont-gomery, T. E. Montroy, J. Nagy, T. Natoli, J. P. Nibarger, M. D. Niemack, V. Novosad,S. Padin, C. Pryke, C. L. Reichardt, J. E. Ruhl, B. R. Saliwanchik, J. T. Sayre, K. K. Schaffer,E. Shirokoff, K. Story, C. Tucker, K. Vanderlinde, J. D. Vieira, G. Wang, R. Williamson,V. Yefremenko, K. W. Yoon, and E. Young. Performance and on-sky optical characterizationof the SPTpol instrument. In Millimeter, Submillimeter, and Far-Infrared Detectors and In-strumentation for Astronomy VI, volume 8452 of Proc. SPIE, page 84521F, September 2012.

[14] J. Zmuidzinas. Superconducting Microresonators: Physics and Applications. Ann. Rev. Cond.Matt. Phys., 3:169–214, 2012.

[15] J. Gao, M. Daal, J. M. Martinis, A. Vayonakis, J. Zmuidzinas, B. Sadoulet, B. A. Mazin, P. K.Day, and H. G. Leduc. A semiempirical model for two-level system noise in superconductingmicroresonators. Applied Physics Letters, 92(21):212504, May 2008.

[16] P. K. Day, H. G. Leduc, A. Goldin, T. Vayonakis, B. A. Mazin, S. Kumar, J. Gao, andJ. Zmuidzinas. Antenna-coupled microwave kinetic inductance detectors. Nuclear Instru-ments and Methods in Physics Research A, 559:561–563, April 2006.

[17] J. Schlaerth, A. Vayonakis, P. Day, J. Glenn, J. Gao, S. Golwala, S. Kumar, H. Leduc,B. Mazin, J. Vaillancourt, and J. Zmuidzinas. A Millimeter and Submillimeter Kinetic In-ductance Detector Camera. Journal of Low Temperature Physics, 151:684–689, May 2008.

[18] S. R. Golwala, C. Bockstiegel, S. Brugger, N. G. Czakon, P. K. Day, T. P. Downes, R. Duan,J. Gao, A. K. Gill, J. Glenn, M. I. Hollister, H. G. LeDuc, P. R. Maloney, B. A. Mazin,

29

S. G. McHugh, D. Miller, O. Noroozian, H. T. Nguyen, J. Sayers, J. A. Schlaerth, S. Siegel,A. K. Vayonakis, P. R. Wilson, and J. Zmuidzinas. Status of MUSIC, the MUltiwavelengthSub/millimeter Inductance Camera. In Society of Photo-Optical Instrumentation Engineers(SPIE) Conference Series, volume 8452, Sept. 2012.

[19] S. Doyle, P. Mauskopf, J. Naylon, A. Porch, and C. Duncombe. Lumped Element KineticInductance Detectors. Journal of Low Temperature Physics, 151:530–536, April 2008.

[20] A. Monfardini, A. Benoit, A. Bideaud, L. Swenson, A. Cruciani, P. Camus, C. Hoffmann,F. X. Desert, S. Doyle, P. Ade, P. Mauskopf, C. Tucker, M. Roesch, S. Leclercq, K. F.Schuster, A. Endo, A. Baryshev, J. J. A. Baselmans, L. Ferrari, S. J. C. Yates, O. Bourrion,J. Macias-Perez, C. Vescovi, M. Calvo, and C. Giordano. A Dual-band Millimeter-wave Ki-netic Inductance Camera for the IRAM 30 m Telescope. Astrophys. J. Supp., 194:24, June2011.

[21] M. Calvo, A. Benoıt, A. Catalano, J. Goupy, A. Monfardini, N. Ponthieu, E. Barria, G. Bres,M. Grollier, G. Garde, J.-P. Leggeri, G. Pont, S. Triqueneaux, R. Adam, O. Bourrion, J.-F.Macıas-Perez, M. Rebolo, A. Ritacco, J.-P. Scordilis, D. Tourres, A. Adane, G. Coiffard,S. Leclercq, F.-X. Desert, S. Doyle, P. Mauskopf, C. Tucker, P. Ade, P. Andre, A. Beelen,B. Belier, A. Bideaud, N. Billot, B. Comis, A. D’Addabbo, C. Kramer, J. Martino, F. Mayet,F. Pajot, E. Pascale, L. Perotto, V. Reveret, A. Ritacco, L. Rodriguez, G. Savini, K. Schuster,A. Sievers, and R. Zylka. The NIKA2 Instrument, A Dual-Band Kilopixel KID Array forMillimetric Astronomy. Journal of Low Temperature Physics, 184:816–823, August 2016.

[22] A. Ritacco, N. Ponthieu, A. Catalano, R. Adam, P. Ade, P. Andre, A. Beelen, A. Benoıt,A. Bideaud, N. Billot, O. Bourrion, M. Calvo, G. Coiffard, B. Comis, F.-X. Desert, S. Doyle,J. Goupy, C. Kramer, S. Leclercq, J. F. Macıas-Perez, P. Mauskopf, A. Maury, F. Mayet,A. Monfardini, F. Pajot, E. Pascale, L. Perotto, G. Pisano, M. Rebolo-Iglesias, V. Reveret,L. Rodriguez, C. Romero, F. Ruppin, G. Savini, K. Schuster, A. Sievers, C. Thum, S. Trique-neaux, C. Tucker, and R. Zylka. Polarimetry at millimeter wavelengths with the NIKA cam-era: calibration and performance. ArXiv e-prints, September 2016.

[23] A. Baryshev, J. Baselmans, S. Yates, L. Ferrari, L. Bisigello, R. Janssen, A. Endo, T. Klap-wijk, B. Klein, S. Heyminck, and R. Gusten. Large Format Antenna Coupled MicrowaveKinetic Inductance Detector Arrays for Radio Astronomy. In IRMMW-THz 39, September2014.

[24] J. Hubmayr, J. Beall, D. Becker, H.-M. Cho, M. Devlin, B. Dober, C. Groppi, G. C. Hilton,K. D. Irwin, D. Li, P. Mauskopf, D. P. Pappas, J. Van Lanen, M. R. Vissers, Y. Wang, L. F.Wei, and J. Gao. Photon-noise limited sensitivity in titanium nitride kinetic inductance de-tectors. Applied Physics Letters, 106(7):073505, February 2015.

[25] N. Galitzki, P. A. R. Ade, F. E. Angile, P. Ashton, J. A. Beall, D. Becker, K. J. Bradford,G. Che, H.-M. Cho, M. J. Devlin, B. J. Dober, L. M. Fissel, Y. Fukui, J. Gao, C. E. Groppi,S. Hillbrand, G. C. Hilton, J. Hubmayr, K. D. Irwin, J. Klein, J. van Lanen, D. Li, Z.-Y. Li,

30

N. P. Lourie, H. Mani, P. G. Martin, P. Mauskopf, F. Nakamura, G. Novak, D. P. Pappas,E. Pascale, G. Pisano, F. P. Santos, G. Savini, D. Scott, S. Stanchfield, C. Tucker, J. N.Ullom, M. Underhill, M. R. Vissers, and D. Ward-Thompson. The Next Generation BLASTExperiment. Journal of Astronomical Instrumentation, 3:1440001, 2014.

[26] B. J. Dober, P. A. R. Ade, P. Ashton, F. E. Angile, J. A. Beall, D. Becker, K. J. Bradford,G. Che, H.-M. Cho, M. J. Devlin, L. M. Fissel, Y. Fukui, N. Galitzki, J. Gao, C. E. Groppi,S. Hillbrand, G. C. Hilton, J. Hubmayr, K. D. Irwin, J. Klein, J. Van Lanen, D. Li, Z.-Y. Li,N. P. Lourie, H. Mani, P. G. Martin, P. Mauskopf, F. Nakamura, G. Novak, D. P. Pappas,E. Pascale, F. P. Santos, G. Savini, D. Scott, S. Stanchfield, J. N. Ullom, M. Underhill, M. R.Vissers, and D. Ward-Thompson. The next-generation BLASTPol experiment. In Society ofPhoto-Optical Instrumentation Engineers (SPIE) Conference Series, volume 9153 of Societyof Photo-Optical Instrumentation Engineers (SPIE) Conference Series, page 0, July 2014.

[27] D. Flanigan, H. McCarrick, G. Jones, B. R. Johnson, M. H. Abitbol, P. Ade, D. Araujo,K. Bradford, R. Cantor, G. Che, P. Day, S. Doyle, C. B. Kjellstrand, H. Leduc, M. Limon,V. Luu, P. Mauskopf, A. Miller, T. Mroczkowski, C. Tucker, and J. Zmuidzinas. Photon noisefrom chaotic and coherent millimeter-wave sources measured with horn-coupled, aluminumlumped-element kinetic inductance detectors. Applied Physics Letters, 108(8), 2016.

[28] H. McCarrick, D. Flanigan, G. Jones, B. R. Johnson, P. Ade, D. Araujo, K. Bradford,R. Cantor, G. Che, P. Day, S. Doyle, H. Leduc, M. Limon, V. Luu, P. Mauskopf, A. Miller,T. Mroczkowski, C. Tucker, and J. Zmuidzinas. Horn-coupled, commercially-fabricated alu-minum lumped-element kinetic inductance detectors for millimeter wavelengths. Review ofScientific Instruments, 85(12):123117, December 2014.

[29] P. D. Mauskopf, S. Doyle, P. Barry, S. Rowe, A. Bidead, P. A. R. Ade, C. Tucker, E. Castillo,A. Monfardini, J. Goupy, and M. Calvo. Photon-noise limited performance in aluminumlekids. Journal of Low Temperature Physics, 176(3):545–552, 2014.

[30] C. McKenney, H. G. Leduc, L. J. Swenson, P. K Day, B. H. Eom, and J. Zmuidzinas. Designconsiderations for a background limited 350 micron pixel array using lumped element super-conducting microresonators. In Society of Photo-Optical Instrumentation Engineers (SPIE)Conference Series, volume 8452, pages 84520S–84520S–10, July 2012.

[31] A. Kovacs, P. S. Barry, C. M. Bradford, G. Chattopadhyay, P. Day, S. Doyle, S. Hailey-Dunsheath, M. Hollister, C. McKenney, H. G. LeDuc, N. Llombart, D. P. Marrone,P. Mauskopf, R. C. O’Brient, S. Padin, L. J. Swenson, and J. Zmuidzinas. SuperSpec: de-sign concept and circuit simulations. In Society of Photo-Optical Instrumentation Engineers(SPIE) Conference Series, volume 8452 of Society of Photo-Optical Instrumentation Engi-neers (SPIE) Conference Series, September 2012.

[32] A. Patel, A. Brown, W. Hsieh, T. Stevenson, S.H. Moseley, K. U-yen, N. Ehsan, E. Barrentine,G. Manos, and E.J. Wollack. Fabrication of mkids for the microspec spectrometer. AppliedSuperconductivity, IEEE Transactions on, 23(3):2400404–2400404, June 2013.

31

[33] Bradley R. Johnson, Daniel Flanigan, Maximilian H. Abitbol, Peter A. R. Ade, Sean Bryan,Hsiao-Mei Cho, Rahul Datta, Peter Day, Simon Doyle, Kent Irwin, Glenn Jones, Sarah Ker-nasovskiy, Dale Li, Philip Mauskopf, Heather McCarrick, Jeff McMahon, Amber Miller,Giampaolo Pisano, Yanru Song, Harshad Surdi, and Carole Tucker. Polarization sensitivemulti-chroic mkids. Proc. SPIE, 9914:99140X–99140X–12, 2016.

[34] Heather McCarrick, Maximilian H. Abitbol, Peter A. R. Ade, Peter Barry, Sean Bryan,George Che, Peter Day, Simon Doyle, Daniel Flanigan, Bradley R. Johnson, Glenn Jones,Henry G. LeDuc, Michele Limon, Philip Mauskopf, Amber Miller, Carole Tucker, and JonasZmuidzinas. Development of dual-polarization lekids for cmb observations. Proc. SPIE,9914:99140O–99140O–7, 2016.

[35] S. W. Henderson, R. Allison, J. Austermann, T. Baildon, N. Battaglia, J. A. Beall, D. Becker,F. De Bernardis, J. R. Bond, E. Calabrese, S. K. Choi, K. P. Coughlin, K. T. Crowley, R. Datta,M. J. Devlin, S. M. Duff, J. Dunkley, R. Dunner, A. van Engelen, P. A. Gallardo, E. Grace,M. Hasselfield, F. Hills, G. C. Hilton, A. D. Hincks, R. Hlozek, S. P. Ho, J. Hubmayr, K. Huf-fenberger, J. P. Hughes, K. D. Irwin, B. J. Koopman, A. B. Kosowsky, D. Li, J. McMahon,C. Munson, F. Nati, L. Newburgh, M. D. Niemack, P. Niraula, L. A. Page, C. G. Pappas,M. Salatino, A. Schillaci, B. L. Schmitt, N. Sehgal, B. D. Sherwin, J. L. Sievers, S. M. Si-mon, D. N. Spergel, S. T. Staggs, J. R. Stevens, R. Thornton, J. Van Lanen, E. M. Vavagiakis,J. T. Ward, and E. J. Wollack. Advanced ACTPol Cryogenic Detector Arrays and Readout.Journal of Low Temperature Physics, March 2016.

[36] R. Datta, J. Hubmayr, C. Munson, J. Austermann, J. Beall, D. Becker, H. M. Cho, N. Halver-son, G. Hilton, K. Irwin, D. Li, J. McMahon, L. Newburgh, J. Nibarger, M. Niemack,B. Schmitt, H. Smith, S. Staggs, J. Van Lanen, and E. Wollack. Horn Coupled MultichroicPolarimeters for the Atacama Cosmology Telescope Polarization Experiment. Journal of LowTemperature Physics, 176:670–676, September 2014.

[37] S. W. Deiker, W. Doriese, G. C. Hilton, K. D. Irwin, W. H. Rippard, J. N. Ullom, L. R. Vale,S. T. Ruggiero, A. Williams, and B. A. Young. Superconducting transition edge sensor usingdilute AlMn alloys. Applied Physics Letters, 85:2137, September 2004.

[38] A. E. Lowitz, A. D. Brown, V. Mikula, T. R. Stevenson, P. T. Timbie, and E. J. Wollack.Design, Fabrication, and Testing of a TiN/Ti/TiN Trilayer KID Array for 3 mm CMB Obser-vations. Journal of Low Temperature Physics, 184:627–633, August 2016.

[39] H. Surdi. Applications of Kinetic Inductance: Parametric Amplifier & Phase Shifter, 2DEGCoupled Co-planar Structures & Microstrip to Slotline Transition at RF Frequencies. PhDthesis, ARIZONA STATE UNIVERSITY, 2016.

[40] B. R. Johnson, P. A. R. Ade, D. Araujo, K. J. Bradford, D. Chapman, P. K. Day, J. Di-dier, S. Doyle, H. K. Eriksen, D. Flanigan, C. Groppi, S. Hillbrand, G. Jones, M. Limon,P. Mauskopf, H. McCarrick, A. Miller, T. Mroczkowski, B. Reichborn-Kjennerud, B. Smi-ley, J. Sobrin, I. K. Wehus, and J. Zmuidzinas. The Detector System for the Stratospheric

32

Kinetic Inductance Polarimeter ( Skip). Journal of Low Temperature Physics, 176:741–748,September 2014.

[41] D. C. Araujo, P. A. R. Ade, J. R. Bond, K. J. Bradford, D. Chapman, G. Che, P. K. Day,J. Didier, S. Doyle, H. K. Eriksen, D. Flanigan, C. E. Groppi, S. N. Hillbrand, B. R. John-son, G. Jones, M. Limon, A. D. Miller, P. Mauskopf, H. McCarrick, T. Mroczkowski,B. Reichborn-Kjennerud, B. Smiley, J. Sobrin, I. K. Wehus, and J. Zmuidzinas. A LEKID-based CMB instrument design for large-scale observations in Greenland. In Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series, volume 9153 of Society ofPhoto-Optical Instrumentation Engineers (SPIE) Conference Series, page 0, August 2014.

[42] K. D. Irwin, L. R. Vale, N. E. Bergren, S. Deiker, E. N. Grossman, G. C. Hilton, S. W. Nam,C. D. Reintsema, D. A. Rudman, and M. E. Huber. Time-division squid multiplexers. AIPConference Proceedings, 605(1):301–304, 2002.

[43] Piet A. J. de Korte et al. Time-division superconducting quantum interference device multi-plexer for transition-edge sensors. Review of Scientific Instruments, 74(8):3807–3815, 2003.

[44] H. Hui et al. BICEP3 focal plane design and detector performance. Proc. SPIE Int. Soc. Opt.Eng., 9914, 2016.

[45] E. S. Battistelli, M. Amiri, B. Burger, M. Halpern, S. Knotek, M. Ellis, X. Gao, D. Kelly,M. MacIntosh, K. Irwin, and C. Reintsema. Functional description of read-out electronics fortime-domain multiplexed bolometers for millimeter and sub-millimeter astronomy. Journalof Low Temperature Physics, 151(3):908–914, 2008.

[46] T. Essinger-Hileman, J. W. Appel, J. A. Beal, H. M. Cho, J. Fowler, M. Halpern, M. Has-selfield, K. D. Irwin, T. A. Marriage, M. D. Niemack, L. Page, L. P. Parker, S. Pufu, S. T.Staggs, O. Stryzak, C. Visnjic, K. W. Yoon, and Y. Zhao. The Atacama B-Mode Search:CMB Polarimetry with Transition-Edge-Sensor Bolometers. AIP Conference Proceedings,1185(1):494–497, 2009.

[47] D. S. Swetz et al. Overview of the Atacama Cosmology Telescope: Receiver, instrumentation,and telescope systems. Astrophys. J. Suppl., 194:41, 2011.

[48] M. D. Niemack et al. ACTPol: A polarization-sensitive receiver for the Atacama CosmologyTelescope. Proc. SPIE Int. Soc. Opt. Eng., 7741:77411S, 2010.

[49] P. A. R. Ade et al. BICEP2 II: Experiment and Three-Year Data Set. Astrophys. J., 792(1):62,2014.

[50] Z. Ahmed et al. BICEP3: a 95GHz refracting telescope for degree-scale CMB polarization.Proc. SPIE Int. Soc. Opt. Eng., 9153:91531N, 2014.

[51] Thomas Essinger-Hileman, Aamir Ali, Mandana Amiri, John W. Appel, Derek Araujo,Charles L. Bennett, Fletcher Boone, Manwei Chan, Hsiao-Mei Cho, David T. Chuss, Fe-lipe Colazo, Erik Crowe, Kevin Denis, Rolando Dunner, Joseph Eimer, Dominik Gothe,

33

Mark Halpern, Kathleen Harrington, Gene C. Hilton, Gary F. Hinshaw, Caroline Huang,Kent Irwin, Glenn Jones, John Karakla, Alan J. Kogut, David Larson, Michele Limon, Lind-say Lowry, Tobias Marriage, Nicholas Mehrle, Amber D. Miller, Nathan Miller, Samuel H.Moseley, Giles Novak, Carl Reintsema, Karwan Rostem, Thomas Stevenson, DeborahTowner, Kongpop U-Yen, Emily Wagner, Duncan Watts, Edward J. Wollack, Zhilei Xu, andLingzhen Zeng. Class: the cosmology large angular scale surveyor. Proc. SPIE Int. Soc. Opt.Eng., 9153:91531I–91531I–23, 2014.

[52] P. A. R. Ade et al. BICEP2 / Keck Array V: Measurements of B-mode Polarization at DegreeAngular Scales and 150 GHz by the Keck Array. Astrophys. J., 811:126, 2015.

[53] A. A. Fraisse et al. SPIDER: Probing the Early Universe with a Suborbital Polarimeter.JCAP, 1304:047, 2013.

[54] K. D. Irwin and G. C. Hilton. Transition-Edge Sensors. In C. Enss, editor, Cryogenic ParticleDetection, page 63. Springer, 2005.

[55] Shawn W. Henderson et al. Readout of two-kilopixel transition-edge sensor arrays for ad-vanced actpol. Proc. SPIE Int. Soc. Opt. Eng., 9914, 2016.

[56] P. A. R. Ade et al. Antenna-coupled TES bolometers used in BICEP2, Keck array, andSPIDER. Astrophys. J., 812(2):176, 2015.

[57] W. B. Doriese, K. M. Morgan, D. A. Bennett, E. V. Denison, C. P. Fitzgerald, J. W. Fowler,J. D. Gard, J. P. Hays-Wehle, G. C. Hilton, K. D. Irwin, Y. I. Joe, J. A. B. Mates, G. C. O’Neil,C. D. Reintsema, N. O. Robbins, D. R. Schmidt, D. S. Swetz, H. Tatsuno, L. R. Vale, andJ. N. Ullom. Developments in time-division multiplexing of x-ray transition-edge sensors.Journal of Low Temperature Physics, 184(1):389–395, 2016.

[58] W. B. Doriese, K. M. Morgan, D. A. Bennett, E. V. Denison, C. P. Fitzgerald, J. W. Fowler,J. D. Gard, J. P. Hays-Wehle, G. C. Hilton, K. D. Irwin, Y. I. Joe, J. A. B. Mates, G. C. O’Neil,C. D. Reintsema, N. O. Robbins, D. R. Schmidt, D. S. Swetz, H. Tatsuno, L. R. Vale, andJ. N. Ullom. Erratum to: Developments in time-division multiplexing of x-ray transition-edgesensors. Journal of Low Temperature Physics, 184(1):396–396, 2016.

[59] D. Prele, F. Voisin, M. Piat, T. Decourcelle, C. Perbost, C. Chapron, D. Rambaud, S. Maestre,W. Marty, and L. Montier. A 128 multiplexing factor time-domain squid multiplexer. Journalof Low Temperature Physics, 184(1):363–368, 2016.

[60] Christine A. Jhabvala et al. Kilopixel backshort-under-grid arrays for the Primordial InflationPolarization Explorer. Proc. SPIE Int. Soc. Opt. Eng., 9153:91533C, 2014.

[61] J. W. Fowler, W. B. Doriese, G. C. Hilton, K. D. Irwin, D. R. Schmidt, G. M. Stiehl, D. S.Swetz, J. N. Ullom, and L. R. Vale. Optimization and analysis of code-division multiplexedTES microcalorimeters. J. Low. Temp. Phys., 167:713–720, 2012.

34

[62] K. D. Irwin, H. M. Cho, W. B. Doriese, J. W. Fowler, G. C. Hilton, M. D. Niemack, C. D.Reintsema, D. R. Schmidt, J. N. Ullom, and L. R. Vale. Advanced code-division multiplexersfor superconducting detector arrays. J. Low. Temp. Phys., 167:588–594, 2012.

[63] G. M. Stiehl, W. B. Doriese, J. W. Fowler, G. C. Hilton, K. D. Irwin, C. D. Reintsema, D. R.Schmidt, D. S. Swetz, J. N. Ullom, and L. R. Vale. Code-division multiplexing for x-raymicrocalorimeters. Appl. Phys. Lett., 100:072601, 2012.

[64] Tijmen de Haan, Graeme Smecher, and Matt Dobbs. Improved performance of tes bolometersusing digital feedback. Proc. SPIE, 8452:84520E–84520E–10, 2012.

[65] M. A. Dobbs, M. Lueker, K. A. Aird, A. N. Bender, B. A. Benson, L. E. Bleem, J. E.Carlstrom, C. L. Chang, H.-M. Cho, J. Clarke, T. M. Crawford, A. T. Crites, D. I. Flani-gan, T. de Haan, E. M. George, N. W. Halverson, W. L. Holzapfel, J. D. Hrubes, B. R.Johnson, J. Joseph, R. Keisler, J. Kennedy, Z. Kermish, T. M. Lanting, A. T. Lee, E. M.Leitch, D. Luong-Van, J. J. McMahon, J. Mehl, S. S. Meyer, T. E. Montroy, S. Padin,T. Plagge, C. Pryke, P. L. Richards, J. E. Ruhl, K. K. Schaffer, D. Schwan, E. Shirokoff,H. G. Spieler, Z. Staniszewski, A. A. Stark, K. Vanderlinde, J. D. Vieira, C. Vu, B. West-brook, and R. Williamson. Frequency multiplexed superconducting quantum interferencedevice readout of large bolometer arrays for cosmic microwave background measurements.Rev. Sci. Instrum., 83(7):073113, 2012.

[66] Amy N. Bender, Jean-Francois Cliche, Tijmen de Haan, Matt A. Dobbs, Adam J. Gilbert,Joshua Montgomery, Neil Rowlands, Graeme M. Smecher, Ken Smith, and Andrew Wil-son. Digital frequency domain multiplexing readout electronics for the next generation ofmillimeter telescopes, 2014.

[67] D. Hanson, S. Hoover, A. Crites, P. A. R. Ade, K. A. Aird, J. E. Austermann, J. A. Beall,A. N. Bender, B. A. Benson, L. E. Bleem, J. J. Bock, J. E. Carlstrom, C. L. Chang, H. C.Chiang, H-M. Cho, A. Conley, T. M. Crawford, T. de Haan, M. A. Dobbs, W. Everett, J. Gal-licchio, J. Gao, E. M. George, N. W. Halverson, N. Harrington, J. W. Henning, G. C. Hilton,G. P. Holder, W. L. Holzapfel, J. D. Hrubes, N. Huang, J. Hubmayr, K. D. Irwin, R. Keisler,L. Knox, A. T. Lee, E. Leitch, D. Li, C. Liang, D. Luong-Van, G. Marsden, J. J. McMa-hon, J. Mehl, S. S. Meyer, L. Mocanu, T. E. Montroy, T. Natoli, J. P. Nibarger, V. Novosad,S. Padin, C. Pryke, C. L. Reichardt, J. E. Ruhl, B. R. Saliwanchik, J. T. Sayre, K. K. Schaffer,B. Schulz, G. Smecher, A. A. Stark, K. T. Story, C. Tucker, K. Vanderlinde, J. D. Vieira, M. P.Viero, G. Wang, V. Yefremenko, O. Zahn, and M. Zemcov. Detection of B-Mode Polariza-tion in the Cosmic Microwave Background with Data from the South Pole Telescope. Phys.Rev. Lett., 111:141301, Sep 2013.

[68] B. Reichborn-Kjennerud, A. M. Aboobaker, P. Ade, F. Aubin, C. Baccigalupi, C. Bao, J. Bor-rill, C. Cantalupo, D. Chapman, J. Didier, M. Dobbs, J. Grain, W. Grainger, S. Hanany,S. Hillbrand, J. Hubmayr, A. Jaffe, B. Johnson, T. Jones, T. Kisner, J. Klein, A. Korotkov,S. Leach, A. Lee, L. Levinson, M. Limon, K. MacDermid, T. Matsumura, X. Meng, A. Miller,M. Milligan, E. Pascale, D. Polsgrove, N. Ponthieu, K. Raach, I. Sagiv, G. Smecher, F. Stivoli,

35

R. Stompor, H. Tran, M. Tristram, G. S. Tucker, Y. Vinokurov, A. Yadav, M. Zaldarriaga,and K. Zilic. EBEX: a balloon-borne CMB polarization experiment. In Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series, volume 7741 of Society ofPhoto-Optical Instrumentation Engineers (SPIE) Conference Series, July 2010.

[69] KD Irwin and Konrad W Lehnert. Microwave squid multiplexer. Applied physics letters,85(11):2107–2109, 2004.

[70] John Arthur Benson Mates. The microwave SQUID multiplexer. PhD thesis, University ofColorado, 2011.

[71] P. K. Day, H. G. LeDuc, B. A. Mazin, A. Vayonakis, and J. Zmuidzinas. A broadbandsuperconducting detector suitable for use in large arrays. 425:817–821, October 2003.

[72] Jonas Zmuidzinas. Superconducting microresonators: Physics and applications. AnnualReview of Condensed Matter Physics, 3(1):169–214, 2012.

[73] JAB Mates, KD Irwin, LR Vale, GC Hilton, J Gao, and KW Lehnert. Flux-ramp modulationfor squid multiplexing. Journal of Low Temperature Physics, 167(5-6):707–712, 2012.

[74] D. A. Bennett, J. A. B. Mates, J. D. Gard, A. S. Hoover, M. W. Rabin, C. D. Reintsema, D. R.Schmidt, L. R. Vale, and J. N. Ullom. Integration of tes microcalorimeters with microwavesquid multiplexed readout. IEEE Transactions on Applied Superconductivity, 25(3):1–5, June2015.

[75] SM Stanchfield, PAR Ade, J Aguirre, JA Brevik, HM Cho, R Datta, MJ Devlin, SR Dicker,B Dober, D Egan, et al. Development of a microwave squid-multiplexed tes array formustang-2. Journal of Low Temperature Physics, pages 1–6, 2016.

[76] SW Henderson, R Allison, J Austermann, T Baildon, N Battaglia, JA Beall, D Becker,F De Bernardis, JR Bond, E Calabrese, et al. Advanced actpol cryogenic detector arraysand readout. Journal of Low Temperature Physics, pages 1–8, 2016.

[77] BA Benson, PAR Ade, Z Ahmed, SW Allen, K Arnold, JE Austermann, AN Bender,LE Bleem, JE Carlstrom, CL Chang, et al. Spt-3g: a next-generation cosmic microwavebackground polarization experiment on the south pole telescope. In SPIE Astronomical Tele-scopes+ Instrumentation, pages 91531P–91531P. International Society for Optics and Pho-tonics, 2014.

[78] CD Reintsema, J Beall, W Doriese, W Duncan, L Ferreira, GC Hilton, KD Irwin, D Schmidt,J Ullom, L Vale, et al. A tdma hybrid squid multiplexer. Journal of Low Temperature Physics,151(3-4):927–933, 2008.

[79] J. van Rantwijk, M. Grim, D. van Loon, S. Yates, A. Baryshev, and J. Baselmans. Multiplexedreadout for 1000-pixel arrays of microwave kinetic inductance detectors. IEEE Transactionson Microwave Theory and Techniques, 64(6):1876–1883, June 2016.

36

[80] O. Bourrion, A. Bideaud, A. Benoit, A. Cruciani, J. F. Macias-Perez, A. Monfardini,M. Roesch, L. Swenson, and C. Vescovi. Electronics and data acquisition demonstrator for akinetic inductance camera. Journal of Instrumentation, 6:6012, June 2011.

[81] B. A. Mazin. Microwave Kinetic Inductance Detectors: The First Decade. In B. Young,B. Cabrera, and A. Miller, editors, American Institute of Physics Conference Series, volume1185 of American Institute of Physics Conference Series, pages 135–142, December 2009.

[82] O. Bourrion, C. Vescovi, A. Catalano, M. Calvo, A. D’Addabbo, J. Goupy, N. Boudou,J. F. Macias-Perez, and A. Monfardini. High speed readout electronics development forfrequency-multiplexed kinetic inductance detector design optimization. Journal of Instru-mentation, 8:C12006, December 2013.

[83] R. Duan, S. McHugh, B. Serfass, B. A. Mazin, A. Merrill, S. R. Golwala, T. P. Downes,N. G. Czakon, P. K. Day, J. Gao, J. Glenn, M. I. Hollister, H. G. Leduc, P. R. Maloney,O. Noroozian, H. T. Nguyen, J. Sayers, J. A. Schlaerth, S. Siegel, J. E. Vaillancourt, A. Vay-onakis, P. R. Wilson, and J. Zmuidzinas. An open-source readout for MKIDs. In Millime-ter, Submillimeter, and Far-Infrared Detectors and Instrumentation for Astronomy V, volume7741 of Proc. SPIE, page 77411V, July 2010.

[84] S. McHugh, B. A. Mazin, B. Serfass, S. Meeker, K. O’Brien, R. Duan, R. Raffanti, andD. Werthimer. A readout for large arrays of microwave kinetic inductance detectors. Reviewof Scientific Instruments, 83(4):044702–044702, April 2012.

[85] O. Bourrion, C. Vescovi, J. L. Bouly, A. Benoit, M. Calvo, L. Gallin-Martel, J. F. Macias-Perez, and A. Monfardini. NIKEL: Electronics and data acquisition for kilopixels kineticinductance camera. Journal of Instrumentation, 7:7014, July 2012.

[86] O. Bourrion, A. Benoit, J. L. Bouly, J. Bouvier, G. Bosson, M. Calvo, A. Catalano,J. Goupy, C. Li, J. F. Macıas-Perez, A. Monfardini, D. Tourres, N. Ponchant, and C. Vescovi.NIKEL AMC: Readout electronics for the NIKA2 experiment. ArXiv e-prints, February2016.

[87] K. Hattori, S. Ariyoshi, M. Hazumi, H. Ishino, A. Kibayashi, S. Mima, C. Otani, N. Satoh,T. Tomaru, M. Yoshida, and H. Watanabe. Novel Frequency-Domain Multiplexing MKIDReadout for the LiteBIRD Satellite. Journal of Low Temperature Physics, 167:671–677,June 2012.

[88] S. J. C. Yates, A. M. Baryshev, J. J. A. Baselmans, B. Klein, and R. Gusten. Fast Fouriertransform spectrometer readout for large arrays of microwave kinetic inductance detectors.Applied Physics Letters, 95(4):042504, July 2009.

[89] J. Schlaerth, A. Vayonakis, P. Day, J. Glenn, J. Gao, S. Golwala, S. Kumar, H. Leduc,B. Mazin, J. Vaillancourt, and J. Zmuidzinas. A Millimeter and Submillimeter Kinetic In-ductance Detector Camera. Journal of Low Temperature Physics, 151:684–689, May 2008.

37

[90] B. J. Dober, P. A. R. Ade, P. Ashton, F. E. Angile, J. A. Beall, D. Becker, K. J. Bradford,G. Che, H.-M. Cho, M. J. Devlin, L. M. Fissel, Y. Fukui, N. Galitzki, J. Gao, C. E. Groppi,S. Hillbrand, G. C. Hilton, J. Hubmayr, K. D. Irwin, J. Klein, J. Van Lanen, D. Li, Z.-Y. Li,N. P. Lourie, H. Mani, P. G. Martin, P. Mauskopf, F. Nakamura, G. Novak, D. P. Pappas,E. Pascale, F. P. Santos, G. Savini, D. Scott, S. Stanchfield, J. N. Ullom, M. Underhill, M. R.Vissers, and D. Ward-Thompson. The next-generation BLASTPol experiment. In Millimeter,Submillimeter, and Far-Infrared Detectors and Instrumentation for Astronomy VII, volume9153 of Proc. SPIE, page 91530H, July 2014.

[91] L. J. Swenson, P. K. Day, C. D. Dowell, B. H. Eom, M. I. Hollister, R. Jarnot, A. Kovacs,H. G. Leduc, C. M. McKenney, R. Monroe, T. Mroczkowski, H. T. Nguyen, and J. Zmuidzi-nas. MAKO: a pathfinder instrument for on-sky demonstration of low-cost 350 micron imag-ing arrays. In Millimeter, Submillimeter, and Far-Infrared Detectors and Instrumentation forAstronomy VI, volume 8452 of Proc. SPIE, page 84520P, September 2012.

[92] S. R. Golwala, C. Bockstiegel, S. Brugger, N. G. Czakon, P. K. Day, T. P. Downes, R. Duan,J. Gao, A. K. Gill, J. Glenn, M. I. Hollister, H. G. LeDuc, P. R. Maloney, B. A. Mazin,S. G. McHugh, D. Miller, O. Noroozian, H. T. Nguyen, J. Sayers, J. A. Schlaerth, S. Siegel,A. K. Vayonakis, P. R. Wilson, and J. Zmuidzinas. Status of MUSIC, the MUltiwavelengthSub/millimeter Inductance Camera. In Millimeter, Submillimeter, and Far-Infrared Detectorsand Instrumentation for Astronomy VI, volume 8452 of Proc. SPIE, page 845205, September2012.

[93] A. Catalano, M. Calvo, N. Ponthieu, R. Adam, A. Adane, P. Ade, P. Andre, A. Beelen,B. Belier, A. Benoıt, A. Bideaud, N. Billot, N. Boudou, O. Bourrion, G. Coiffard, B. Comis,A. D’Addabbo, F.-X. Desert, S. Doyle, J. Goupy, C. Kramer, S. Leclercq, J. F. Macıas-Perez,J. Martino, P. Mauskopf, F. Mayet, A. Monfardini, F. Pajot, E. Pascale, L. Perotto, V. Reveret,L. Rodriguez, G. Savini, K. Schuster, A. Sievers, C. Tucker, and R. Zylka. Performance andcalibration of the NIKA camera at the IRAM 30 m telescope. Astron. and Astroph., 569:A9,September 2014.

[94] M. Calvo, A. Benoıt, A. Catalano, J. Goupy, A. Monfardini, N. Ponthieu, E. Barria, G. Bres,M. Grollier, G. Garde, J.-P. Leggeri, G. Pont, S. Triqueneaux, R. Adam, O. Bourrion, J.-F.Macıas-Perez, M. Rebolo, A. Ritacco, J.-P. Scordilis, D. Tourres, A. Adane, G. Coiffard,S. Leclercq, F.-X. Desert, S. Doyle, P. Mauskopf, C. Tucker, P. Ade, P. Andre, A. Beelen,B. Belier, A. Bideaud, N. Billot, B. Comis, A. D’Addabbo, C. Kramer, J. Martino, F. Mayet,F. Pajot, E. Pascale, L. Perotto, V. Reveret, A. Ritacco, L. Rodriguez, G. Savini, K. Schuster,A. Sievers, and R. Zylka. The NIKA2 Instrument, A Dual-Band Kilopixel KID Array forMillimetric Astronomy. Journal of Low Temperature Physics, 184:816–823, August 2016.

[95] B. A. Mazin, P. K. Day, K. D. Irwin, C. D. Reintsema, and J. Zmuidzinas. Digital readoutsfor large microwave low-temperature detector arrays. Nuclear Instruments and Methods inPhysics Research A, 559:799–801, April 2006.

38

[96] D. C. Araujo, P. A. R. Ade, J. R. Bond, K. J. Bradford, D. Chapman, G. Che, P. K. Day,J. Didier, S. Doyle, H. K. Eriksen, D. Flanigan, C. E. Groppi, S. N. Hillbrand, B. R. John-son, G. Jones, M. Limon, A. D. Miller, P. Mauskopf, H. McCarrick, T. Mroczkowski,B. Reichborn-Kjennerud, B. Smiley, J. Sobrin, I. K. Wehus, and J. Zmuidzinas. A LEKID-based CMB instrument design for large-scale observations in Greenland. In Millimeter, Sub-millimeter, and Far-Infrared Detectors and Instrumentation for Astronomy VII, volume 9153of Proc. SPIE, page 91530W, August 2014.

[97] J. J. A. Baselmans, J. Bueno, S. J. C. Yates, O. Yurduseven, N. Llombart, K. Karatsu, A. M.Baryshev, L. Ferrari, A. Endo, D. J. Thoen, P. J. de Visser, R. M. J. Janssen, V. Murugesan,E. F. C. Driessen, G. Coiffard, J. Martin-Pintado, P. Hargrave, and M. Griffin. A kilo-pixelimaging system for future space based far-infrared observatories using microwave kineticinductance detectors. ArXiv e-prints, September 2016.

[98] A. Ritacco, N. Ponthieu, A. Catalano, R. Adam, P. Ade, P. Andre, A. Beelen, A. Benoıt,A. Bideaud, N. Billot, O. Bourrion, M. Calvo, G. Coiffard, B. Comis, F.-X. Desert, S. Doyle,J. Goupy, C. Kramer, S. Leclercq, J. F. Macıas-Perez, P. Mauskopf, A. Maury, F. Mayet,A. Monfardini, F. Pajot, E. Pascale, L. Perotto, G. Pisano, M. Rebolo-Iglesias, V. Reveret,L. Rodriguez, C. Romero, F. Ruppin, G. Savini, K. Schuster, A. Sievers, C. Thum, S. Trique-neaux, C. Tucker, and R. Zylka. Polarimetry at millimeter wavelengths with the NIKA cam-era: calibration and performance. ArXiv e-prints, September 2016.

39